Cracking open 25th torsos
There are a bunch of different methods out there for breaking apart the torsos of 25th styled figures. Which one will work best for you will come down to a matter of preference.
Richard Woodhouse, the designer who first showed us how to do it at the 2007 GI Joe Convention, uses either a wood or plastic block shaped like this.
He sets the point of the block under the armpit of the figure, with the arms out straight to the sides, and whacks the figure with a rubber mallet. He was able to do this in one whack and it took about 1 second.
You can see this technique demonstrated in the following video:
Wrench and Socket/Allen Wrench bit
For this method you will need an adjustable wrench and either a socket or a hex-head Allen wrench bit.
Darkwynter and Apacolypse popularized the wrench and socket method. They attach a socket and piece of cardboard to opposite ends of an adjustable wrench with duct tape. The pressure created by squeezing of the wrench pops the torso apart. You can see a more thorough version of Apacolypse's tutorial here.
Some people have found the sockets to be frustrating because the roundness of the socket makes it hard to balance the figure on. Chief suggested using a hex shaped Allen wrench bit instead. He also uses gauze instead of cardboard because it better forms to the shape of the figure.
I use a razor blade. Start under the arm and work your way up. Do this on both sides. Then I slide a small flat head screwdriver (or any metal thing dull and flat) then pry it open. I've done this for 3 torsos without hurting any of them.
X-acto slicing method
Others have found success simply using an x-acto knife to cut the torsos in half. Be careful not to cut so much that you end up with a noticeable gap between the torsos. A dulled blade is also recommended.
Matthew/Hasbro hybrid method
Score the seem of the torso with an x-acto knife. Jam a small flat screw driver into the arm pit with the arms out straight. Then whack the screwdriver with a small hammer. The torso will pop apart.
Some people swear by freezing the torsos first before cutting, prying, whacking, or squeezing. Others swear by boiling the figures first. Both of these weaken the bond that holds the torsos together.