There's probably a lot of books on the subject, from people like Paul Stead, you could do worse than spend some money on books.
Really though it's a case of checking other peoples examples and trying to learn from them.
I strongly advise you get a program called Deep Exploration, it's great for traditional texturing, like all it really is is a model viewer and converter, but I would have it running on a second display while the primary display has the art package on it - it's a great tool for viewing your texturing work while it happens, like you save in your art package then you can instantly see the results in Deep Exploration.
Often with guns, I would use a layer or 2 for guides, 1 layer with the UV wireframe, and one extra layer for lining things up, just little lines here and there, to use as a guide when drawing textures. It helps you to orientate yourself as well, on your texture a piece could be inverted, rotated, or just awkward to work with - drawing some guides onto it will show how it looks when textured onto the mesh. It's always a good idea to plan out your texture first, by drawing your details in flat colour first so you can check them easily in Deep Exp' before drawing them properly.
There's a lot of material online for drawing textures, but I think that it'd be easier to work with a book, it's annoying trying to texture and read a tutorial at the same time, unless you can print off lots of tutorial pages instead.
We're going down... in a spiral to the ground...
