Two key components of expert skiing are balance and flexibility. The good news is that you can work on these when you are not on the hill. If you don't have a copy of Ski Flex you might want to pick one up.
I like to stretch at least 2x per day all year long. In addition I use my staircase to simulate deep angles starting from the 3rd stair up and moving down to the bottom stair. I started with the staircase early this season and it helped in my ability to drop my hips into a turn while keeping a relatively upright upper body.
Some devices to consider are a 16" Wobble Board, Bongo Board, the 8Board, and Harb Carvers.
A Skier's Edge is a good training tool for balance especially if you have the slope simulator attachmentand its good training for flexing and tipping. It also gives you an aerobic workout and works your ski leg muscles.
HH has also mentioned biking as a great off season supplement to skiing.