I'd say the reverse is probably better advise - don't even think about moving up to the magnetometer until you see all of your possessions advance through the X-ray machine. Many TSA agents actually announce that instruction now (I suspect a liability issue at play?) but it's a wise precaution.

One thing I'd add to Heather's tips - if you're coming from/going to a colder climate and have bulky clothing on (a sweater, for example), be prepared for TSA to pat you down. You CAN opt to take your sweater off and run through the x-ray, but the pat-down is the faster option - and believe me, the TSA agent doesn't want to do it any more than you want it done to you.

@Johnny Jet - the shoe thing seems to vary by airport - some airports still like shoes in the bin. The best thing is to listen to the TSA barkers at the checkpoint - they'll tell you whether to keep shoes on the belt or put them in the bin.

One last thing - for anyone who travels with a computer frequently, I can't even begin to extol the virtues of an 'x-ray friendly' laptop bag. It seems like a small thing, but not having to take your computer out makes the checkpoint experience go much faster. I splurged on a Brenthaven bag with a 'butterfly' design, and even with the butterfly zipper open, I can still keep the bag on my shoulder while I ready my other items and then just lay it flat on the belt. On the other side, you just zip up and go. It's the little things...