Big mountain is judged on 5 things.
1. Line Choice-the line you descend down the venue, airs and cliffs help build up your line score
2. Technique-this entails things like making round turns, keeping your hands up, not being in the back seat...etc
3. Control-things like back slaps, putting a hand down, or any major bobble will cost you in the control category. to obtain a good control score you just have to have a good, clean, run
4. Fluidity-this is basically just how fluid you are down the venue. minimum hesitation, and maintaining good speed are what the judges look for in this category.
5. Style and Energy-judges want to see skiing energetically down the venue making the whole run exiting and fun to watch. tricks also help in this category
The head judge awards the riders line score, and then all three judges give their scores for the other four categories. The scores in these four categories aren't allowed to be more than 3 points above the line score.
Here's an example of a big mountain venue: http://the-north-face-masters.rmvn.stackablehost.com/wp-content/uploads/2012/03/Silverfox-Aprilfb1.jpg
And here's an example of a run someone might do on that same venue (skip to 1:16: https://www.youtube.com/watch?v=TVdguDkOUKQ
“Rather than an unnecessary and predictable story about a bunch of assholes partying, consider this a recitation of the notion that skiing still draws camaraderie from the reckless, decadent, and fun loving antics that it was founded on.” – Tom Warnick