First see if there are any discounts you qualify for in terms of the accomodations or tickets. For instance, booking through costco or AAA can save you quite a bit.
One fun and inexpensive way to stay at Disney World is to car camp in the Fort Wilderness Resort. It will cost you about $50 a night to pitch a tent.
Staying at a non-Disney owned hotel is usually a little cheaper than staying inside the park. Most nearby hotels offer a free shuttle service to the park, so a far walk shouldn't be a deterant. You will probably spend most of your time seeing the sights and attractions, so spending less on a room you'll hardly be seeing is a good idea.
Food inside the park can be very expensive, so if you don't mind the walk, head a block or two away from the park and have your choice of cheaper restaurants.
Disney also offers all-inclusive packages that include hotel, food, and tickets, but these tend to be pricey. They are cheaper than if you bought them separately, but still expensive.
There is so much to see at Disney World that splurging a little may be in order.