For any generic helpful tips, check out the burn and dodge tools for shading the surfaces and adding depth to the texture. Its one of the most basic things about making skins and textures, so you can create wrinkles and cracks and all kinds of stuff with that. Its all about shading.
Another thing that is really helpful is using plenty of layers and getting familiar with the different blending options. Like I added the finer details to the skins, the cloth´s pattern etc, with just new layers that are semi-transparent.
And, yeah. I´d suggest to check out the layers, their blending options, the burn and dodge tools.http://skindom.planetquake.gamespy.com/tutorials/
those might be helpful also, dunno. Dont be afraid to contact me on MSN if you have that, I´d be happy to help ya out.