Change the contrast/brightness/levels/etc... so that the head you paste over the picture looks like it has the same lighting. Also zoom in and really cut out those extra little pixels around the edges that aren't a part of the head.

There is a lot more as well but I'm not sure what program you are using or how detailed you want to be. For example, if you're using Photoshop, get creative with the smudge, burn and blur tools. Slightly blurring the edges helps it look more natural as well.