Depends on the production quality you want to get out of your work. If you are just filming skiing and don't necessarily care if your edits are of the highest production quality, you can get by with your kit lens and something like the DIY fig rig mentioned above.
If you are looking at seriously getting into making films other than ski edits, or want to produce ski edits with any real production value, that is a whole other story. If this is the case, the bare minimum would be a decent tripod for stabilization, ND filters to control depth of field without cranking your shutter, and an external mic or external sound recorder. Beyond that, you would want to start looking into a set of fast manual focus prime lenses (old slr lenses work great and can be had for pretty cheap), some sort of handheld stabilization (glidecam, shoulder rig, etc), a beefier tripod with a real fluid head, a slider, follow focus, lighting kit, the list is basically infinite.
