Important part for HR texture work is finding decent quality stock or free images you can work with. I mostly just hit up google and/or deviantart. There's a bloke on deviant that has some interesting metal textures up for instance that are free for nonprofit use (drop him a line though showing what you're using them for), he's over here: http://wojtar-stock.deviantart.com/
Other then that I could give you some pointers but the rest just comes down to a time-sink and lots of trial and error:
- Try to avoid repeaters or noticeable mirror effects with metal scratches or cloth materials.
- Make sure your patterns are seamless on large surfaces AKA if you have to repeat a texture multiple times that has a pattern on it, make sure it lines up right.
- Put diffuse shading back when re-texturing materials, otherwise you run risk of your mesh looking very flat. Even with the specmap, normalmap and ingame lighting, the diffuse shading is there for a reason and often helps cover up texture stretching and wrapseams plus helps suggest depth.
- Group your layers per material, makes it easier to work with the specmap later.
- Finish your diffuse then build your specmap up from the layered project. Makes it easier to separate elements and manipulate their contrast/brightness per specmap channel.
Aside from that tools of the trade are optimally Photoshop due to it's versatility and a tablet (even if it's a small one).
The pen tool is utter sex in Photoshop, even if you have a tablet, use it. Alot. Learn to use layer masks for brightness/contrast/colour. Separate your shading from your material layers, it saves time. While you can do it with free software such as Gimp and with a mouse, you just won't have a toolset as powerful so that will affect the quality of your end product.
Size, don't overdo it, while full body armor looks best at 4096, not everything needs to be this big and even though we have the option to permanently replace assets doesn't mean it doesn't strain a system when you do so choose wisely. Always keep in mind that your one mod might not cause issues on it's own but if everyone would just upscale when unnecessary it starts to become an issue real fast.
ONe other tip, I highly recommend loading your mesh up in 3ds, or whatever poison you use and to load the DDS diffuse that you are working on. That way when you make changes to your texture and save it to DDS you can instantly see the effect of your changes in your 3D program rather than having to take it ingame and testing it out with texmod every time. As for the spec, I'd take that one ingame for sure but building the diffuse up outside is a lot better for your workflow.