There's no substitute for experience with a particular airline at a particular airport, so hopefully others will contribute some.
Start checking once every two weeks or so whenever you wish, just to get a feel. My limited experience suggests the best prices appear 2-4 months out for off-season fares. For very busy times buying further in advance might be a better strategy. If you are looking at any airline that allows you to see seating charts before purchase (Delta, Northwest, etc) that can help a lot. If there are still a lot of open seats you don't need to buy in a panic.
Is money everything? Are you willing to drive 2-3 hours just to save $30 per ticket? I'm not. Nonstops are priceless, as they are much less susceptible to delay. Good flight times are also something to think about. If a flight arrives in the evening instead of the morning, you've lost at least half a day at WDW. That's worth something too.
