My husband says I am a bit of a gear freak when it comes to ski clothing, but I truly believe that the right gear makes all the difference in your happiness and comfort. It has taken me a while to figure out what works best for me.
Apres ski clothing is easy. Breck can be really cold after dark, but it is (thankfully) casual. I usually bring one pair of jeans and one pair of decent-looking yoga pants that I can wear in public, 2-3 sweaters (including what I wear on the plane) and a pair of silk long-underwear to put underneath at night. I don't ski in the silks. I never leave home without at least one pashmina scarf. I prefer to have two or three hats, preferably fleece-lined that cover my ears, and a pair of mittens if only because it gives my gloves a chance to dry.
For skiing, I adore my soft-shell pants. They aren't as warm as a hard shell or my insulated pants, but they are super comfortable and stretchy. I wear a pair of insulated compression tights underneath (CW-X brand) in a capri length so as not to add another layer inside my boots. If it is especially cold, I might layer a pair of Hot Chillys or a thicker pair of fleece tights underneath my ski pants. In my opinion, you can skimp a little and buy less expensive ski pants (as long as they are waterproof, its fine). There is a Christy's outlet in Frisco across from Walmart and the Main Street Outlet in downtown Breck have some great deals on last season and overstock clothing.
In rental ski boots, you might want a medium-weight smartwool sock. If you own boots, go for the thinnest ski sock you can find. Thick socks do not equal warmer feet. In fact, it is the opposite.
On my upper half, a good sports bra is a must. I usually do one or two layers of thin Hot Chillys and a fleecy mid-layer. I've found that the expensive fleece ones aren't any better than the cheap ones from Old Navy - the only difference is that the neck zipper area may have more protection from chafing. I do like a mid-layer with thumb holes though. Then I put on an insulated jacket - I only bring the one. I also use a fleece neck gaiter which I can pull up over my nose on the lift chair.
I can't imagine not having boots out there. You don't need the Sorel brand (though, I just bought a pair at The Clymb for 50% off!), but a decent waterproof lug-sole boot is a must. Work boots would be sufficient - even from Walmart.
Other musts: Goggles for on-mountain, sunglasses for off, good sunscreen - I like the Dermatone line Z-Cote tin, which is a high SPF and water-free, so it doesn't freeze, lip balm, and like an earlier poster said, a helmet and the best gloves you can afford. I love my brimmed helmet - it is far warmer than a hat and my goggles rarely fog when I wear.