i spent a season there 2 years ago. here's my suggestions on what to expect/do:
-the different ski areas/domains are pretty dispersed - you will have to take lots of buses around. try not to get pissed about that.
-everything is extortionately expensive, from food and drink to liftpasses. this is totally down to the name of the resort and nothing to do with what you're getting. it has some of the most antiquated lift infrastructure you will ever see. try not to get pissed about that.
-the different ski areas (5 of them i think) are laying at different altitudes and different exposures and can be as much as 45 minutes away from each other. this means you should check the forecast for the next day(s) and plan where you will go accordingly. south facing areas will get nice and sunny in the morning and turn to slush around 12/13 o'clock to the point of being unrideable. north facing areas will be icy in the mornings but may get some sunshine as the sun goes further west in the afternoons making them best in those after lunch.
-go to the mountain guides lodge/main ski school lodge for the best/most up to date information on weather and mountain conditions. befriend the mountain guides there who are basically freelancers and pump them for information. don't expect to find many who snowboard though. this lodge/office is in the centre of town Chamonix, just up from the main tourist office.
-be smart about going off-piste. quite a few of the ski areas have glaciers and that also means crevasses. its an awe and respect inspiring mountain range (i guarantee unlike anything you've seen before) and if you have any sense it will make you shit yourself even thinking about going off-piste unguided. so, don't do it! get a mountain guide if you want to go off-piste and otherwise stick to groomers.
-if you can, do the vallee blanche (guided run). its a 20km run from 3800m down a glacier to the bottom of the valley, if you get a bluebird day. it may not be the best run but it will be an extraordinary experience, part of which is going up in a gondola that goes almost vertically up to a massive station built into jagged cliffs at the peak which is at 3900m.
-Grand Montets is a nice ski area but my absolute favorite was Flegere-Brevent. That place was a lot less crowded, lots and lots of steep runs, and getting suntanned all day.
-There is a very good and almost totally unused DC snowboard park in Les Houches.
-the little chinese place (if it still exists) opposite the cinema in the centre of chamonix town makes excellent food! it was so good i enjoyed that more than fondues and other local food.
days not on snow in 2011: 11