I want to post a couple general comments, then offer a couple tips.
A lot of the results depends on a person's specific style of play, including specific party composition and member development. I play all female (except near the beginning when you don't have all four females, of course). No problems even on Nightmare (aside from the broken Tactics and specific bugs, etc, but that's true for any difficulty and is not related to strategy and/or tips, per se). In addition, I only play on Hard or Nightmare in order to obtain the Friendly Fire concern (very silly of Bioware to not include that as an option on Options screen rather than attaching it to difficulty settings).
Despite what some people claim, including the official guide, I am happy to state that NO, you do not need to have the stereotypical party of tank, DPS, healer, etc, and especially not in Easy or Normal mode (and I'd even add Hard mode, myself, as this approach not being necessary at all). Even for Nightmare mode, you don't really need to worry much about tanking, per se. Morrigan and Leliana can handle almost anything if handled properly and strategically.
Likewise, I can say that no, using distance spells and various other skills as they are intended to be used is not cheating, exploiting, or anything underhanded. On the contrary, it is roleplaying the characters as they are meant to behave according to having those specific skills and related approach to solving problems/conflicts. Likewise, having a warrior decked out in the heaviest armor with tons of magical protections plow through everything is not cheating, exploiting, or anything else remotely underhanded. These and other examples are allowed by the game because all of them are different approaches to resolving conflicts and obstacles. After all, how else should we roleplay characters who have such skills? Avoiding roleplaying in a roleplaying game or claiming that roleplaying according to certain skill sets is somehow "cheating" or "exploitative" makes no sense.
As for specific tips (according to my play style, character development, and party compositon, mind you), I can offer a couple pointers.
1) Grab everything and sell anything you don't need in order to generate income, especially during the first half or so. This includes stripping everything off NPC who will die or leave the party (for example, Jowan and Lily don't need their equipment once you lead Jowan to his phylactery, and the NPCs who join you for the Joining are going to die, anyway, so they certainly don't need theirs). The economy in this game, like many modern RPGs, is horribly broken and unrealistic, so don't let it get you down or encourage you to waste your very limited resources.
2) Crank out basic "Lesser" health and lyrium poultices. Keep your stock around 90-100 of each. In addition, keep a "resource stock" (elfroot, lyrium dust, and lots of flasks) of around 200-400 each (more of the flasks since they are used for both as well as other stuff such as poison). Why only the "Lesser" even on Nightmare? Well, because the cost/benefit ratio for the higher level potions is incredibly poor, and you'll actually pick up a fair number of the higher items anyway. Of course, near the end game, you may want to make some of the higher potions, but even then you should be able to limit it due to the ridiculously unbalanced cost/benefit of the resources needed to make them (plus the recipes, of course). [see tip #10 below]
3) Buy all tomes and use them before the Landsmeet, but probably just before, as they are not really needed earlier.
4) Do all possible optional/side quests, especially any that result in "bonus" items that are very useful for your PC. For example, the "Key to the City" in Orzammar is a quest and item that gives you +2 on all attributes when equipped.
5) In many encounters, especially ambushes, Hold Position is your friend! This is especially true in the latter part of the game when enemies often have traps laid all over the place. Just pay attention to your party and help them if needed, even taking control of someone if necessary. For example, if Morrigan is surrounded and taking abuse, and if she has her next Cold chain spell (Cone of Cold) after her initial two she always has, switch to her and freeze the enemes solid! Having Wynne's Tactics (and Lelianna's, eventually) set to cast Stone Fist on any enemy who is immobilized is quite helpful.

In areas where Ohgren has to be a party member, I have had him take out 3 enemies in one sweep after freezing them. Same thing applies to combos like Grease and Fireball - set the Grease, aggro enemies with a long distance AoE, arrow, etc, and blast them as they run into the Grease. The game gives combo spells for a reason - use them!
6) Max or near max (at least level 3) lines of primary abilities (spells/talents) or you will regret it greatly later when the enemies have maxed abilities. Base notice: you should be able to max 3 chains of spells/talents plus your 2 specializations, and still have a couple of points to spare. Plan ahead or suffer.
7) Don't hesitate to switch equipment so that characters have maximized their capabilities at any point.
8) Go after personal quests and give gifts so as to max approval of party members, thus granting max "plot bonus" rewards for stats. For example, Morrigan gains small bonuses to Magic, Lelianna gains on Cunning, etc. Max the plot bonuses in order to max the potential of your party.
9) Top of Tactics list, place a check for "Self: Health < 25%" and take a "least powerful" health poultice if true. Also, do the same on next line for mana (with lyrium potion). Finally, be sure to set Options for Quick Heal to "least powerful" in order to avoid consuming powerful healing by accident.
10) Do Broken Circle first after Lothering. Be sure to gather all possible "Essence" boosts for attributes during the Fade section of the quest. In addition, this gains Wynne in your party ASAP. By the time she goes up a couple levels, you will not need to make many lesser healing poultices; the usage and rate of finding new ones plus her healing abilities (if set up properly) will keep you clear most of the time.
11) Dog will find a very nice "Piece of Wood" that serves as a powerful staff during your time in Lothering. Get it as your first primary weapon of choice because it's better than most anything else you can find (or buy, even if you had the gold, which you do not). You can replace it later with the staff from the Elder Oak in the forest, as well as Harrowmont's staff (if you side with him as Orzammar's new king).
Well, that's enough for now, I think.