Hi Holiday Lover
I think the advice form IceandFire is spot on.
If you travel in large groups then leaving it to last minute isn't advisable as last minute places tend to be for couples or small groups, if you are a large group then book early to take advantage of early bird deals and free group places.
If you have no preference of resort, accommodation, and you are travelling as a couple then you could leave it as late as the day before you travel in some cases (I used to work for an online ski agent and we would have last minute deals come in and people booking the day before travel), a lot of late deal availability will also depend on snow conditions, this year the deals were not as plentiful as the year before when snow was pretty poor the deals were amazing and there were loads!
As mentioned by iceandfire to get a cracking deal start your search about 2 weeks before you want to go and try going through an on-line agent as they receive deals on a daily basis and can offer a wide range of offers including chalets, self catered, hotels and b&b, plus whole packages and just accommodation only deals, or try chalets direct, this is a great site to find a deal as you can post in their solution room you exact criteria and the chalet companies will contact you with their offers.
Another point to mention about last minute deals is that is you find exactly what you are looking for at the right price BOOK IT, don`t wait as last minute deals book fast and if you don`t snap it up someone else will.
January and April I would say are the best months to get the best deals, for guaranteed snow go in January and you never know you may get yourself such an amazing deal that you may just be able to squeeze in a 2nd last min deal in April