Stand on a non carpeted floor, touch something other than the PC to discharge yourself, then try to stay in contact with the case to prevent static buildup. If you use a non conductive table surface and floor you probably won't have any problems anyway after you discharge any static.
Work on a carpet'ed surface while wearing socks. Make sure to move your feet against the carpet very quickly while working.
Kidding asdie. Working on a hard surface (non-cloth). Wear rubber sole'ed shoes and touch some kind of metal surface before you start. I've been modding computers for a while now and have yet to have any problems. Most importantly, just be smart.