I would just like to make a couple of suggestions:
1. Lighting. Its a good idea to try and match lighting in both intensity and direction, so as to make the pic look a little less "cut and paste"
2. Feathering tool. In most paint programs there is a feathering tool which helps to take away the sharp edges when blending one picture with another, i.e.; one head on another body, etc.
3. Size and Proportion. Nuff said.
I think your well on the way. Your efforts are admirable, but if you're trying to accomplish a more polished look, you might try these things. Keep up the good work.