I had a lot of trouble figuring out UV mapping, too… here are a couple of things I found helpful:
You generally need to put seams on some of the edges. Imagine trying to unfold the model yourself if it was made of cardboard, and where the edges would have to be in order to unfold it until it was mostly flat. ( Mesh > Edges > Mark Seam ).
Use UVs > Scripts > Save UV Face Layout to output the outlines of your unwrapped mesh into a graphic file. In fact, you can start by loading that same file as your unwrapped texture, to see where the faces end up!
Hope this helps, and I think your beat up cube looks pretty cool! Try playing with the lighting, though; I usually use a three-point lighting rig for my test renders.
Is it merely a problem of unwrapping or rather of making the texture?
You also might want to try the new Smart Projections feature sometime, it makes pretty awesome unwraps of “simple” meshes automatically. Obviously it gives you less control to actually determine the UV Face layout but still, for simple textures it is quite nice.
Other than that, it’s all about practice, mechanical / blocky things are usually not that hard to unwrap with well placed seams, as sarah said (point 2 is also quite important, you will definitely need to export the Layout to be able to do a fitting texture in a 2D Editor!).
Yes… you can have multiple unwrappings, and I think you can map multiple images to each… I remember someone (PapaSmurf?) talking about this in another thread not too long ago (which is where I first found out about it). You might find it with a quick forum search…
In UV Face select mode, you can click the “New” button next to where it says “UV Texture” in the “Mesh” panel. That lets you make multiple unwrappings if you need them, I think.
To Map different images to different textures, I think you set the texture to be an image texture, set the mapping to “UV Mapping”, and put the name of the mapping into the “UV” Field near the mapping button.
I hope this helps… I haven’t tried it myself, though!