I heard, and it seems everyone else has, that you should always wear the retainer.
Put it in listerine, or some other strong mouthwash every week. It gets rid of germs and helps fight the icky nasty goop that'll rub off your teeth onto it.
Also, I heard brush it once daily. Don't put it in napkins, those accidentally get thrown away a lot.
I don't even have mine, but the other day in the orthodontist, the kid next to me was getting his xD