My best single tip for everyone is to grab a largish ziploc bag on your way out the door when you leave home, and keep it in your pocket or purse.
Once you get in line, take out the bag and have everyone drop in keys, pocket change, watches, wallets that are carried in pockets, sunglasses, cell phones, pagers, PDA's, and any large pieces of jewelry; anything metal normally carried on one's person. (Be sure to take your ID out of wallets and keep it on you.) Close the bag and put it inside the outer pocket of one of your carryon bags before you reach the scanner. Before you leave security, check to see that the ziploc bag is undisturbed, but then proceed to your gate. Once you get there you will have plenty of time to put everything back where it belongs. Please do NOT stop to put all your stuff back into pockets, etc. on the other side of the security gate; it backs up the security line, as people behind you cannot come through until the area is clear.
Children's pockets should be checked before you leave home or your car; they often grab things at the last minute and slip them in their pockets; what they grabbed might not be allowed on a plane. (A favorite pocketknife, or a toy gun, for example. Even rubber knives are forbidden on planes.)
As Carol noted, pay attention to what you are planning to wear to travel. Avoid bulky clothing, and any clothing with metal trim, metal buttons, or lots of zippers. Leave your belt inside your carryon if at all possible. Have everyone wear slip-on shoes if they are bringing them.
We always have a really annoying situation at STL on flights before major holidays. There is a military prep school ~100 miles w. of here, and the kids' uniforms feature STEEL chains and insignia. They are required to wear their dress uniforms when they travel home for the holidays, and it always takes them forever to untangle and unfasten all of that metal to get through the security gates. (They also wear steel-toed combat boots, of course.) I've taken to checking the website to see when the last day of class is. If I'm flying the next day, I give myself an extra hour.