When in a group, I always shout out, "STANDING" a few strokes before I stand up. Gives them time to back off my wheel. I ride sometimes with a guy who stands up on the slightest of inclinces, and he really throws the bike back a bit when he does - I know to give him plenty of room.

A bit of advice an experienced rider gave me on one of my earliest centuries: stand up whenever and as often as possible. Those little breaks and changes of position will help out your legs and bottom over the course of a long ride. I've used his advice often!