Normally people buy the tickets at full price at the gates of Disney world. However, there are some very easy tricks to get the tickets at discounted prices, and save some money:
1) Look for tickets on craigslist
2) Try to find tickets on ebay
3) Check out some local coupons
4) Find the currently running offers

I found an interesting article that details the above simple methods to get tickets at highly discounted prices:
http://SaveFewBucks.Blogspot.com