(This advice has been given to me in the past by a very well known pilot around here!)
You could offer to help the cabin crew tidy the cabin during the turn around. Even if they don't take you up on your offer it will put you in their good books - and you won't be left standing there feeling like a spare part during the (often very hectic) turn around.
Probably the most important point has already been raised by flapsforty. It is important to learn the flight's call sign before you take off, and to listen out for it during conversation. (It's quite a knack to be holding reasonably intelligent conversation whilst monitoring maybe two different frequencies on the radio!)
If you've been told not to talk until 10,000', then thats probably what you should do. However, the airline I have been with recently operates a "sterile flight-deck" until 20,000' - which seems more realistic.
There isn't really a lot more to it, the rest is very often common sense. By that I mean things like greeting the other members of the crew and letting them know you appreciate their hospitality etc.
Above all, enjoy it! It is a reasonably rare, and fantastic opportunity. I don't think I'll ever tire of riding on the jump seat - that is of course, until I sit in the right hand seat!
TPuk