Anyone have any ideas about how to create that "trembling" effect seen in many anime movies? I'm thinking it's a simple 2 frame cycle, but how different and which parameters change within those 2 frames?

and what is it that should be trembling?

A person/character. That shaking effect when someone is nervous or anxious, for instance...

That is keyframe-animation ;) You can try animating the twist values at a high frequency and see the result.

I will probably do it in Photoshop instead of in Poser but will try it in 3D as well.

Why Photoshop? If you had a compositing program like After Effects you could render the character seperate from the background and animate the position manually to give you the jiggles. Actually Premiere, Final Cut Pro and other non-linear editing software can do it as well. In Poser I'd probably try animating the body or the hips. I'd set keyframes where the tremble began and ended to make sure I kept my character in the right place, then open up the graph editor and just move the desired attribute (up and down somewhat randomely along the graph between the beginning and ending points. It would be pretty quick and painless that way.

Thanks bobasaur.

they sometimes do it in a face close-up, by switching the eye highlight slightly back-and-forth.