With our classes we typically only stretch before we start. That includes some joint warmups -- wrist, shoulder/arm, waist, knee, ankle circles, that sort of thing. Then we do a variety of stretches such as the ones karatekid1975 mentioned. As you'd imagine, this is done so that we'll be less likely to pull a muscle when we start practice. Once in a while we do a second round of stretching, either after the beginning stamina training portion of the class or at the very end before we go home. Most of the times we don't though. Personally, I don't think we spend enough time stretching. Yesterday we did the whole set in 15 minutes. But this is also part of the reason I normally get to class a bit earlier, so I have time to do some additional stretching on my own before we start and do some stretches we don't do together as a group.
As for how to stretch, I suppose what's best depends on the person. I think the two main options are stretch and hold or slowly bob in and out of the stretch. I do a combination. Some stretches are easier than others, so the ones that are harder I will usually ease into by bobbing in and out of it, slowly increasing it until I get down where I want to be. Normally I'll hold a stretch for a little while, then release it and do it again. This seems to work pretty well for me. I like holding rather than only bobbing, but I don't want to do just one hold and that's it, so I hold a bit, release, and repeat. When you're bobbing though, you have to be careful not to do it too hard or too fast or you can hurt yourself. It's got to be gentle and slowly increase the stretch. Also as Yari said, in the winter when your body's colder you have to be more careful. In the summer when your body's warmer, it's a bit easier to work the stretches. We're supposed to be working on that during class this summer for that reason, but the training leaders seem to forget that point most of the time. I don't usually do a lot of stretching outside of class, partly because my natural flexibility is pretty good. I may start trying to do some though, because I hurt my hip or something last week while stretching. It was minor and got better quickly enough, but I guess I'll have to be a little more careful when doing the stretching. Doing some on my own on non-class days may help, too.
