In the Store: Know where, what, and how you're skiing, and make sure the shop guy knows. And don't be swayed by brand name or graphics. If possible, demo a few different pairs and go with whatever works.
Guys: Don't lie about your ability. If you're an intermediate skier, buy intermediate-level skis.
In the Field: Most people, especially Western skiers, don't get their skis tuned and waxed as often as they should. Every 12 ski days or so, get your skis tuned by a good shop. Or buy Staying in Tune ($13.50; skiandboardtuning.com), by Jim Mutch, and learn to maintain your boards yourself.
At Home: Treat your skis right in transit: Get a large, rolling (and padded) ski bag like Dakine's Fall Line Double.
If possible, carry your boots—if the airline loses your luggage, you can rent good skis, but it's nearly impossible to rent good boots.
When the season ends, wax your boards and undo the DIN pressure on your bindings so the spring doesn't wear out.