Over pitching is pretty hard to do, unintentionally. It generally occurs when you pitch on top of an old yeast cake.
Common results of over pitching include reduced ester formation, faster fermentation times (due to less growth), thin body and potential off flavors from unhealthy yeasts.
Best bet is to chose an online yeast calculator you like, and try to pitch the target cells. Wyeast offers one, and you have yeastcalc and MrMalty too to determine the optimal pitching rate based on beer gravity and volume.