Ok,
so i'm working with Photoshop Elements, but i'm sure for the other graphic editors it'll be similar.
Start with a basic texture. it's just the base color maybe with a little noise.
next you stamp the UVs on this texture, so you know where everything exactly is. I extracted the lines and put it in an separated layer, so i can always lay it over the latest progress.
If you stamp it on the currently used texture, confirm the overwrite question, but deny the next one that asks for applying the newly created texture to the model. Just minimize XSI and restore, than it's automatically updated.
That way you can easily use the same texture file for multiple model parts, because you can see where you already placed UVs.
Next you go to property>rendermap. Select the resolution of your texture file and choose your UVs. Further i activated super sampling. I have no idea what it is good for exactly, but i think it makes better results.
Choose the Ambient Occlusion map and if you like you can activate Coverage in alpha, that way you get a mask of the UVs in the alpha channel. Press Regenerate Maps.
Make sure you have every other models around your currently selected one visible.
Copy the result texture to your basic texture and set the layer property to multiply
in the next step you should add 2 settings layer for the brightness. One makes it brighther, the other darker.
paint edges etc brighter/darker the way you like. I recoment to choose a non solid brush like this
next you can paint cracks on a new layer. You can use a layer settings to add a "glow" effect, so the outlinings are brighter.
now you can paint dirt to grooves and corners. Play around with the layer properties. I'm using multiply, but that's up to you. but you should use some fuzzy brushes like this
You can use the same for scratches.
In the last step you'll do the bump map. Start with a grey basic and paint the edges and scratches darker, parts that should be higher need to get brighter.
For the specular you can use your final texture, make it grey scale and increase the contrast.
Final result:
next i'll add some colors etc.
On this picture you can see how important the last steps with the scratches, dirt, highlights,.. are.
Body, helmet and backpack are final, but the addon stuff have only the ambient occlusion with the basic texture and you can see that there is something missing. So don't stop after the first few steps!!
Profi tip: if you are really great in XSI you can make reall High polyed models and generate a normal map from the differences to the low polyed one. But i never made something like this.
