I'm not going to endorse the 'butter' method but Gecko is right on. Here is how I do it if it gets too tight:

Using your left hand hold the torso firmly between your thumb and index finger.
Using your right hand, put your thumb under the solid visor piece that sticks out from the helmet and your index finger on the back.
At the same time, tilt (put force on) the top of the torso upwards and the Helmet backwards/down.
The force of your left hand will keep the head from sliding off the neck too easily and the force of your right hand - tilting the helmet back/down is the best for dislodging it from the cavity.

It's become second nature to me and I never have a problem.

-A

I guess that sounds like it would work, I'll give it a shot later. Why is my idea being ignored though?

Because nobody likes you. Buck up! j/k

-A

Lol! Armothe, you are very unexpected!

He he, good one Armothe!

A little refinement to my technique. With the head in question all the way down into the SM armor, all you should have to do is twist the helmet. There is a little slopey part on the armor right below the neck, that the chin guard of the helmet fits right into. Basically, when you twisl the helm, the slope pushes the helm up, and physics does the rest. It's actually pretty simple. I find it easier than Armothe's technique. (I couldn't figure it out...)

I just read this topic right now and Im almost laughing really loud listening to everyone argueing and just all this stuff is funny. . But back on topic, I usually just put the head on a different torso and pull and it comes off.

WOW!...Armothe mate, never seen you come out of your shell...nice work...and you idea pleasently surprised me..it works, all the time, it's awesome i love it

-S

What shell?

His ABS plastic shell of course!

I suppose you can put the head on your newest minifig torso.