First off, I'll admit that I'm no expert on ski fashion. I've never even been skiing. Heck, I can count on one hand the number of times I've even been around enough snow to have a real snowball fight.
However, lack of snow experience aside, I'm going to risk looking ridiculous and offer the following piece of advice:
When packing for a day on the slopes, steer clear of (legless) knitted onesies.
I don't care how beautifully cabled they are, they're simply not practical. Even if you do pair them with knee-high fur-things.