Tandem - should I?

I seem to be incapable of being the stoker. I don't seem to be able to relax properly on the back. It is important that the captain keeps the bike upright. I'm not really sure how to do the whole stoker thing, but whenever I try, the bike gets about 5metres and then wobbles uncontrolably. Not good.

having never rid a tandem, my advice is probably about as useless as the advice of someone who's never been on a bicycle. That being said, Sheldon (and others) have said it is often easier to get a bit more relaxed as a stoke if you hold on to the captain's hips/waist, rather than the bars (which, I guess, makes you very tense and encourages you to try and steer).

What size is the tandem you've got? Maybe it would be a good idea for me and my girlfriend to try and do a few laps of a park here in London before embarking on our long ride home.