I think it was Lasse Andersson sensei that showed us one way, at a seminar suring 2002 here in Sweden. Let's say your right arm is trapped behind your back ("chicken wing").
Grab your right elbow with your left hand, and push towards your ribs. The right elbow should be so tight against your body, that you couldn't push your Visa card in between
Otherwise you might get your shoulder hurt.
Move your body, almost pivoting to the right. Bend over (hehehe!) and move your back under ukes arms and push with your back against ukes elbow (watching out for his knee, so he doesn't smash your face with it). Push up so that uke goes up on his toes (think udosai (sp?) ). Now quickly continue the motion to the right (about 270 degrees would do it) under ukes arms, and uke will loose his grip. Move away from uke and maintain zanshin.
This is a bit hard at first, and my explanation probably makes it worse
. Eitherway, try to start the movement BEFORE uke has your arm caught in the position he wants it in. Otherwise it's damn near impossible.
Atemi to ukes foot works ok aswell. As does a "reverse headbutt".
And as Mike said, why get stuck there in the first place...
PS. Anyone else got any ideas?