While there's an GameFAQ on how to properly build Arcane Warriors (and possibly differing builds on the same subject), I think I'm gonna apply my two-cents here for the mage side of things, and the Arcane Warrior is, by far, the most OP mage spec, especially when coupled with Blood Mage. On that, here we go!
Stats
As mentioned in the starting post, Magic is THE MOST CRUCIAL stat for a mage, as it determines the spellpower of your mage (Spellpower = Magic - 10). As an Arcane Warrior, more magic is even better, as it slightly improves your attack rating (seems like it's every 5 Magic = 1 attack as an Arcane Warrior), determines your melee damage rather than Strength while Combat Magic is active, and allows you to wear medium, heavy, and massive armor sets. However, you don't want to just dump every point into Magic; a little bit of Willpower can go a long way, as well (larger manapool and higher mental resistance FTW), and once you hit about level 10 or so, you can start pumping your Constitution so you can use your health more effectively in Blood Magic mode. Other than that, if you plan on going dual-wield with a MH weapon (such as Spellweaver or the Veshialle) and a dagger in the off-hand (and this is completely viable if you're playing full-party), you might want to hit up some Dexterity to be able to equip higher-tier daggers; other than that, and some Cunning for Coercion and other skills, everything else can be left alone.
Strength - base
Dexterity - base (+some if you want really good daggers as an Arcane Warrior, depending on what equipment you wear)
Willpower - 1 point every level until about 10, then only more from equips (as Arcane Warrior/Blood Mage); otherwise, 1 point every level period
Magic - MAX
Cunning - 16 to get Master Coercion, then up to perhaps 20 for dialogue
Constitution - 2 points every level after 10 until your naked Constitution = naked Willpower, then 1 point every level after (Arcane Warrior/Blood Mage); otherwise, base level
Specializations
Mages can play many different roles throughout the entire game. I feel that the Shapeshifter spec could've been a LOT more powerful, but from what I heard, there was a glitch where the mage's Magic score didn't translate into each shape's Strength score, so boo on that. If you're looking to be a utility character, you will definitely want the Arcane Warrior specialization; this puppy can allow you to do many things, from tanking to being a healer that never dies, to being a CC/DPS powerhouse in both melee and magical combat. To be more of a tank or a healer/support character as an Arcane Warrior, it's usually recognized that you can indeed take Spirit Healer as your second specialization. On the other hand, for soloing purposes and just outright PWNing everything in sight, it's usually recognized to take an Arcane Warrior/Blood Mage build. The thing about the Arcane Warrior is that it often builds to be nearly indestructible in combat, but once the sustained spells you take are active, your mana pool will be more of a mana puddle; Blood Mages compromise this by allowing them to cast from their health, along with access to Blood Wound and even Blood Control, if you feel evil. All in all, a very powerful all-around build.
On the other hand, you can also be a caster of sorts and spec Spirit Healer/Blood Mage. The Spirit Healer tree is great for getting off heals when you need it, and you can still supply some very potent attacks, if you wish. And, of course, when you run out of mana, switch on Blood Magic and keep going at it. The Spirit Healer/Blood Mage aspect is very cool, as Cleansing Aura will be able to pop you back to health if you have to switch Blood Magic off and slam a Lyrium Potion down to keep going. I'm not sure how viable this build is compared to an Arcane Warrior/X build, but I've heard success stories of it, and with how I have Wynne specced on my Arcane Warrior/Blood Mage file, she keeps healing for a long time.
Spells
Four different schools of spells, combined with your mage spells/talents, those of your specializations, and even the addition of the Power of Blood school with the Warden's Keep DLC make mages a somewhat peculiar class to build on. However, through months of research, there is a very nice Arcane Warrior shell that allows the mage to pretty much become indestructible, especially while wearing massive armor. This is outlined below:
Arcane Warrior
Combat Magic -> Aura of Might -> Shimmering Shield -> Fade Shroud
Mage
Arcane Bolt -> Arcane Shield
Primal
Rock Armor
Entropy
Weakness -> Paralyze -> Miasma
This allows for some very interesting stuff to happen; Rock Armor provides even more armor, Arcane Shield increases your defense, and Miasma decreases the attack and defense of all enemies within its radius. What does this mean? You can melee more effectively, and your enemies will have trouble hitting you, inflicting little damage if they do! Shimmering Shield is more of a safety button if you're starting to take some major damage, such as during a boss fight, and Fade Shroud further increases your defense while Combat Magic's active, making an Arcane Warrior a tough little cookie.
Now, what is commonly done with the Arcane Warrior is to not just spec into Blood Mage for its abilities, but to also spec in a way where you can effectively provide CC and dish out major damage. Very nice spells for this set include Mind Blast, Force Field, Glyphs and Crushing Prison, alongside Blood Wound from the Blood Mage spec. If you're soloing, you can also take the cold spell chain in the Primal school, allowing for more CC options while you deal damage. Speaking of dealing damage, Arcane Bolt doesn't do badly at all as a spell that can be cast with weapons drawn, and while in the face of mobs, any of the weapon enhancement spells (Flaming Weapons, Frost Weapons, and Telekinetic Weapons) can add on some significant damage, depending on the situation at hand.
Now, if you're planning on being more of a tank/healer/support-style player, you might not want to look into doing too much magical damage and look at doing more melee damage. In this case, the Spirit Healer spec will definitely get the job done, taking spells up to Lifeward, along with Heal and perhaps the Haste chain in the Creation school. This will definitely make it tough for enemies to effectively damage you, but it doesn't really allow to be able to control whole groups of mobs unless you go for the Paralysis Explosion combo (Glyph of Paralysis + Glyph of Repulsion). This is why it's often more viable to stick to the Arcane Warrior/Blood Mage build; Blood Wound is such a powerful spell, effectively throwing down an AoE Crushing Prison on top of your already-lethal supply of spell and melee might.
As for the ranged magic/support build, I can't really say; if you want to be more of a utility mage, some of the same spells the Arcane Warrior can take should also fit into this mold. However, this also opens up the possibility of creating some potent spell combos, such as Entropic Death or Storm of the Century. It's never bad to experiment, but again, to give you the all-around working, it seems Spirit Healer/Blood Mage will carve things right out for you; just go with what you feel is the most fun to your liking, in this case.
Skills
I find that a character's skill set doesn't seem to come up a lot here, but in certain instances, I find that it can provide some different experiences on each playthrough, so keep this in mind. Now, for the PC, there's a certain skill shell that should work for mages all over (with set beginning skills in bold):
Herbalism
Combat Tactics
Coercion -> Improved Coercion -> Expert Coercion -> Master Coercion
Combat Training -> Improved Combat Training
If you're very good at keeping aggro off of you as a ranged mage, you shouldn't need higher than Improved Combat Training; while you might not think you need it at all, extra mana regen for a mage should NEVER be overlooked. Also, to explore all dialogue options, Master Coercion and a little extra Cunning on the side should do very nicely. As for what else to fill in? Well, personally, I wouldn't mind taking up to Improved Survival for the sake of completing the quest "Elora's Halla" in the Dalish Camp; other than that, it depends on how you play. If you decide to go the Arcane Warrior route, you should definitely put at least one point into Poison-Making; with one point in that and Zevran in your camp, capable of making some very potent poisons, you can add even more damage to your melee swings. Otherwise, if you really don't feel like having spells go poof on you, perhaps extra Combat Training wouldn't be bad to take.
Gear
Ah, yes, now we get to the GOOD stuff. So how should you gear up your mage? Again, that all depends on how you specced them. I can definitely tell you that, as an Arcane Warrior, you can definitely go for massive armor. In fact, make sure you get the Warden Commander Armor set if you have the Warden's Keep DLC; this is most definitely the best armor you can get for a DPS Arcane Warrior. For tanking, I haven't seen any real problems with perhaps trying out the Cailan's Armor set that you can obtain from the Return to Ostagar DLC, but you can change it up however you like according to the situation. Obviously, as an Arcane Warrior/Spirit Healer, you'll probably want to grab a shield; something like the Fade Wall should do very nicely for that aspect, or you can also go with the Cailan's Arms set (Maric's Blade + Cailan's Shield). As a DPS Arcane Warrior, you'll definitely want a main-hand weapon and a dagger to go with it. The best main-hand weapon you could possibly get, from what I've seen, is the Veshialle from Bodahn Feddic, but other great weapons include Starfang (Longsword) and Spellweaver (obtained in the Urn of Sacred Ashes quest); the daggers you can use will depend on if you decide to pump your Dexterity to use the higher-tier ones. If you do, daggers such as the Rose's Thorn, Duncan's Dagger, and the Dead Thaig Shanker are very awesome to behold; otherwise, you might have to look at sticking to something small, such as the Enchanted Dagger or the Thorn of the Dead Gods (Grey Iron from Barlin in Lothering). Either way (AW/SH or AW/BM), you'll also want to take accessories such as the Key to the City, Andruil's Blessing and some of the DLC rings to boost your stats even more, along with the Helm of Honnleath out of the Stone Prisoner DLC. Just keep in mind that, as an Arcane Warrior, some spells can be cast with weapons drawn, while others require either free hands or a staff to be cast quickly; because of this, some AW players decide to carry around a staff in their second Weapon Set slot to switch to it easily. I haven't found that it was necessary for me, but whatever floats your boat.
As a squishy mage, you'll be looking at dressing to what you'd expect from a normal mage: robes, headwear, a staff, and some accessories to go with it all. From what I've seen, you can definitely go with the First Enchanter's Robes, bought from Cesar in the Denerim Market District if you decide to complete the Trial of the Crows initiated from Master Ignacio. If you're going for a DPS/CC mage, I do believe there's a very powerful staff one can obtain from either the Circle Tower Quartermaster or the Tranquil Propietor in the Wonders of Thedas shop; it increases some beneficial stats, as well as almost all of your elemental damages to boot. If you're going more as a healer/support, you may just want to keep gear that increases Magic and Willpower and keeps you on a low fatigue level. Some of the equipment given above might be a bit unnecessary for a normal mage, so if you're just looking to pump Magic, Spellpower, Willpower, etc., there are some other equips that will get the job done.
So let's break this down into certain builds!
Arcane Warrior/Blood Mage DPS/CC
Head: Helm of Honnleath
Body: Warden Commander Armor
Hands: Warden Commander Gloves
Feet: Warden Commander Boots
Neck: Amulet of the War Mage (+5% all elemental damage, considering you'll be using poisons, seems pretty nice)
Belt: Andruil's Blessing
Ring 1: Key to the City
Ring 2: The Lucky Stone
Weapon Set 1: The Veshialle/Spellweaver/Starfang + dagger (again, this depends on your Dex)
Weapon Set 2: (optional staff here)
Arcane Warrior/Spirit Healer Tank/Heal/Support
Head: Helm of Honnleath
Body: Warden Commander Armor
Hands: Warden Commander Gloves
Feet: Warden Commander Boots
Neck: The Spellward/(something that will increase Willpower will work here)
Belt: Andruil's Blessing
Ring 1: Key to the City
Ring 2: The Lucky Stone
Weapon Set 1: Maric's Blade + Cailain's Shield/Spellweaver + Fade Wall
Weapon Set 2: (optional staff here)
I'll see if I can work up a suitable normal mage attire real soon.