I found acetone quite helpful. Put some on a Q-tip and then move the head until you find an open spot. If not, use the Xato-knife. Once you get a good spot where the head is coming apart from the body, use the Q-tip to get inside and soak it in acetone. Try not to get the acetone in the paint, like I did once xD The symbols and eyes come off REALLY easy. It always help ease the process, unless it was like Starswirl I beheaded last night where the plastic seemed melted and took FOREVER T___T Then I had trouble with the hair. -le sigh- But now she's ready for painting. With the acetone, sometime the glue turns to sticky nasty goo too. Clear goo. :O
If you are doing a custom, try using one of the newer G3's, they seem to have tons less glue and come off very easily. I just carefully slit the glue with a craft knife, in just a small spot to begin with, then pull. If the plastic starts to go white from the strain on it, cut a little more and pull again. Soon you will have a headless pony
I use an x-acto (or however you spell that) knife and then brute strength. >__> Try not to cut if off entirely, though, don't forget it has the piece that keeps it on. You don't want to fully decapitate it. XD Trust me, I've seen this done.