If you are going to be at Keystone for more than 5 days, buy a pass soon. They stop selling them at a certain point. You can buy them over the internet. You can have your pictures done the first day you ride. I think a day ticket is normally about 90 dollars. There are discount lift tickets you can get from shops and grocery stores, but I don't think the discounts are normally that good. You may be able to find them for about 10 dollars cheaper.
This is a link to the Summit Value Pass. This is the cheapest vail pass. If you buy them in the summer they are cheaper. Also before you buy the children's passes check out Keystone's website, this season if you stay at certain places you get free lift tickets for children.