Just be a good painter: sand, prime, paint, clear-coat. Don't skip any steps, read the instructions on the paint for how long to wait between coats, etc. Use a paint that can withstand huge temperature changes and a lot of flexing (read the cans or ask the guy at the store).
Here's the trouble though: No matter how well you do it, it will still chip very easily. The good news is when it chips you can always sand it down and paint again, but there just is no good way to protect it that will last. The best way to preserve it is to do a lot of clear coat. The stuff for cars works best. The way to get this is to go to your local car painting shop and leave your skis with them for a couple days, and ask them to use the last few drops in the sprayer after each car on your skis.
I saw a really cool dsign once that leads me to this suggestion: if you do an urban kind of design that incorporates cracks and chip, then when your paint chips it will just enhance the deign.