Let's see if I got this right. Feel free to edit the list, I don't know if I got i all right.
1. Keep mesh under 3000 poly. (depends on the game)
2. One object - One texture, for each object you have you can only use one texture file. Don't use uv-set's only uv-map's.
3. Triangulate your mesh.
4. If you want your character to hold something, model each hand as seperate objects.
5. Do not keyframe the ik, if you use it, keyframe the bones!
6. Do not move your model about in the scene, when doing a walk cycle, make him walk on the spot.
7. Use smoothing groups/softnharden edge when possible.
8. Use time with your uv's, textures does everything. Plan your uv-set, and try to fill the whole square.
9. Don't use larger than 512*512 texture, and let it be square size.
- Kjetil