Organize clothing by layers and pack them systematically. Start with a base layer of long underwear, follow with a middle layer of breathable long-sleeved shirt or sweater, include a fleece coat or vest for insulation, and finish with a ski parka and waterproof pants (or one-piece suit). For a trip of more than two days, include extra inner-level clothes. Garments with more than one use, like tops that can serve different purposes in different weather situations, or clothes that are suitable for both ski and apres-ski use, will lighten your load.
Take inventory of your extremities. Then pack appropriate covers for each one, i.e., hat, helmet, lip protection, face mask, sunscreen, goggles, neck wear, gloves or mitts (don't forget liners), two or more pairs of ski socks, and hand and foot warmers.
Only you know whether a leisure suit, jeans and sweater, or a prom formal works best for you off the slopes. But do have something warm to don at the condo, and take footwear that is non-slip and waterproof, because Aspen gets icy at night. Slippers and sleepwear top off this category.
Besides plane tickets, the only critical items to pack are your boots, skis and poles. That said, renting equipment at a ski area makes travel easier. A soft boot bag may fit under your seat, and can be stuffed with other articles. Skis should be transported in a hard case. Label all bags with your name, phone number and address.
Reservations and Tickets
Keep all your records and proof of flights, hotels, lift tickets, lessons and rentals on your person. Keep them in a money belt.
For most oversights, there are ways to get by, but medicines, money, special dietary items and glasses/contacts are difficult to reproduce.