If they go all pretzel-y, turn off IK and return all the bend/twist/side parameter dials for the affected part to 0 and start over. I use IK only for fine-tuning--putting the hands right where I want them, getting the hip bend/rotation just right without sending the feet flying. Limbs often have to be posed in increments to get them where you want them to go--first shoulder, then forearm, then hand (or bettock, thigh, shin and foot). Lots of people forget to pose the buttock--a must if you're bending legs/hip much more than a few degrees. Dang, I should do a tutorial.