I got stuck in part where I should prepare uv-map the way that if someone else looks at it, he can easilly see which part of mesh is at which part of uv-space. I will go with tank as example again. Let's say, I would try to place uv-islands of the tank body on the bottom part, while uv-islands of his turret would be located at top part. Sounds easy but, I'm not sure how should I go about it. I'm working in Blender, so from as much I know for now, even if I've placed uv-islands where I want manually, margin between them would stop existing, and it would cause problems for texture padding. "Pack Islands" is going to place them back randomly on the uv-space and, I never heard about inside tool or addon for Blender that could at least tell user about the distance between 2 uv-islands. Or at least try to "pack islands" while trying to keep their position.
I'm aware that explaining it could take a lot of your time, so I would apprecipate even some recommendations for a tutorials (even paid ones) that are showing and explaining, how it is done.