Well, I try not to think about it. I lump drinks into my "other expenses" category (other being pretty much everything that isn't hotel, tickets, airfare and food/tips). My strategy, which hasn't failed me yet, is to save my money for all of those things, and then save way more than I need for everything else. I try to make it so that I have so much money for our trips that I even come back with a lot.
If you think this is going to be tough to do in the time alotted, then I recommend drinking as little as possible. Drinks in WDW are ridiculously overpriced. I'll pay the premium for the fun, but not if it's going to put me out.