Definitely a strapless bra. I'd use a convertible one, so you could wear it other places too. I've tried on this style and it's really comfy:
http://www.victoriassecret.com/ss/Satellite?ProductID=1265499209451&c=Page&cid=1305797375056&pagename=vsdWrapper
Get one in black if you're going to so you can't see it!
