You can get better with practise, you get to know what looks good and mess around with photoshop more, there are lots of effects you can use, applying layering effects and such. Those I posted are ones I made for a competition that was held here a while ago. I made more but they weren't very good.

Here are some random wallpapers/collages I've done...



All I did, and all I ever do, is find dozens of images to use and test different ones, moving them around the frame, resizing them, changing the layer opacity and the layer blending, adding effects, putting in pattern and gradient layers, text sometimes, until it looks 'enough' to me. Too much and they look bad.