Dress For Success
First off, empty your pockets (coat and pants). Spare change, keys, and any metal frippery that may be jangling around in your pockets will set off the metal detector and invite extra screening. If you can, don't wear any body piercings through the metal detector. Jewelry that sets off the alarm is cause for additional screening—in private if necessary. Belts with metal clasps also set off the alarm (and must be taken off at the checkpoint), so wearing an outfit without a belt could make things a little easier.
Since shoes must be kicked off during screening at U.S. airports, remember to wear easily removable ones if possible. For more on this, see 9 Things Not to Wear on a Plane.