Emphasis on speed:
* A skier like a bullet down a slope.
* A skier like a white blur through the trees.
* A skier as swift as a falcon in flight.
Emphasis on grace:
* A skier as graceful as a dancer on ice.
* A skier like a feather gliding through the air.
* A skier with the elegance of a swan.
Emphasis on skill:
* A skier as precise as a surgeon.
* A skier like a master puppeteer controlling their body.
* A skier as confident as a seasoned mountaineer.
Emphasis on experience:
* A skier as familiar with the mountain as a sailor with the sea.
* A skier like a seasoned traveler navigating the slopes.
* A skier as comfortable on the mountain as a fish in water.
The best simile will depend on the specific context and the specific image you want to evoke.