When doing plyometrics, you have to be a tad more careful because injuries can occur easier. For example, one can bench in a plyometric fashion by using a smith machine. You'd pile on some plates that you are confortable lifting and plyo them. When the bar is coming back down onto your hand, you have to make sure your elbow is NOT locked. Else you can injure yourself. Even if you do not feel pain, having your elbow locked puts too much stress on it and can weaken it a lot.
Personally I do all my plyo work right before the specific workout. Let's say I just got done squatting, and now I am going to bench. I head to the smith machine, put on little weight and plyo it. And I keep adding plate after plate. Once it is too much for me to plyo, I go over to a bench and start where I just left off but I now bench that weight normally.
I would say that plyometrics is something to do every once in a while. When you are starting, I would not recommend plyometrics, as it is a bit trickier. Once you are doing them, do them every two or three work-out sessions.
P.S. The most popular plyometric movements are push-ups (a.k.a. clapping push-ups), pull-ups (yes, they can be done), and benching.
Roek