hehehe... I am a bridesmaid (or is it bridesmatron?) about two weeks after my baby is due! I am going to get "high compression shapewear"
It sounds like the trick is to wear the shape wear all day everyday (and ideally at night) RIGHT after you have the baby... and up to 8 weeks postpartum. That way it will "squeeze" out the extra fluid retained in your belly. The regular belly bands don't cover enough surface are to have enough compression to do much... so a regular "girdle" will do a better job.
I am thinking of getting this one:
http://www.leonisausa.com/products/strapless-compression-...
or
http://www.vedettestore.com/eng/details.php?id=106
Then you'll have to wear them... all the time...(basically)... most people seem to recommend getting two- so while one is in the was, you are wearing the other one. If you just try to wear one the "day of" you'll just look like a pregnant lady in a girdle LOL (from what I have heard)
Good Luck- I hope it works for you (and for me too!)
-M.