Great advice here! A few other thoughts on BC skiing. Note that I ski in the East, and rarely above treeline, so my tips are more about staying warm and comfortable. We teach a backcountry workshop every year (with dlove's brother, as a matter of fact--he's a good friend of mine). I have Avi 1, but little experience, so I can't offer anything there.
dlove's comment about gloves is spot on. You will get warm on the up, and the idea is to stay as dry as reasonable. I wear a pair of wool liner gloves for the up, then my heavy gloves for the descent. I like Hestra's with the trigger finger--warmth of a mitten with the dexterity of gloves. If it's windy during the climb, I typically also have a pair of softshell gloves by OR that I love--they block the wind but keep your hand reasonably warm. So I tend to carry 3 pairs of gloves--sometimes 4 (2 pairs of liners in case one gets soaked, they weigh nothing). I also carry an extremely warm down jacket that I use during transitions and then keep for emergencies only. It packs into a small pillow size, and fits just fine into my pack. For the weight and warmth, it beats synthetic. I use synthetic (the Atom LT jacket and a hardshell) to ski in. I tend to strip down to my base layer or my base layer with a light microfleece for the up. If it's windy I just put on my hardshell with pit zips open for venting. You want to be just comfortable--if you are really hot on the climb take off layers and open zips.
It's a good idea to carry snacks you can nibble on you, rather than in your pack. You may need energy during your climb, and it's better to have easy access then to stop the whole group and undo your pack. For example, I'll keep a Cliff bar or two in inside pocket and when I think I'll want to eat it I stick it inside my bra to warm/soften it up. Works like a charm.
I have never been able to get water not to freeze in winter using a hydration pack, so I only use water bottles. We've also had them fail and completely soak a pack before, so I go with the safer option. You can either use wool socks to keep them from freezing (if you do this, keep the bottles upside down so the ice forms on the bottom, rather than at the lid), or better yet, insulation sleeves. I keep a bottle in an insulation sleeve afixed to the belt of my pack for easy access during the climb, and put it in my backpack during the descent.
Even if the group is moving fast, BE SURE you eat and drink regularly. Your body needs both fuel and fluid to keep you from becoming hypothermic. It will also keep you warmer. If you notice your extremities getting cold, try eating AND drinking something. And add a layer. When your core starts to get cold, the first thing that you'll notice are cold feet and hands.
Re: transitions, I never carry cheater skins--they blow away and take more time to put back on.
@Pequenita's suggestion to fold them in 4ths as you remove them is a good one--also MUCH easier to separate. My process:
Once at the top, pee if needed. If top is exposed, try to pee in trees or protected area just before the top.
Take off pack, put on down jacket.
Remove skin from ski. If you are on a slight hill, punch poles into the snow below the downhill ski. Then remove skin from uphill ski first. Then the other ski/skin. Place skins in pack.
Tighten boots (I leave them loose for the up); check to make sure my boots are properly connected to my skis. I know people who have lost a ski for this reason and had to recover it in summer.
Put liner gloves in inside pockets (if you can--to keep them from freezing), remove down jacket, put on all my gear for the down, including helmet (always ski with one). Check to make sure all vents are closed. It sucks to fall when your pant vents are open. Ask me how I know.
Tighten straps on pack, make sure nothing is hanging off. Put on pack.
Ski!
I would practice transitions at home--heck you could do it standing in your living room. Also definitely practice kickturns. A kickturn gone bad can be scary.
Sorry, this is all probably more than you need, but it was fun to write. I haven't been BC skiing and I am chomping at the bit. Can you tell?
Enjoy--you will learn a lot!
