We do a lot of what's already been mentioned - especially off-season travel, booking early & taking a certain amount of $$ out of the paycheck each week -right off the top- & put it into a 'vacation fund'. Actually, we have a few funds - travel expenses, food, purchases, etc. We give to them diligently.
Also, we have driven - from upstate NY - the majority of times, which saves a lot of $$ in airfare, but most importantly, allows us to bring bags of food & coolers, which saves us a TON of money in the parks, hotels, & on the road before & after the trip. We bring things like fruit, cereal, granola bars, bagels, cookies, raisins, etc., etc. This really cuts down on the food budget, which can get quite costly, especially when there's a large no. of people. Also, having the coolers to bring into the hotel, we can get milk for cereal or bring food back with us from a restaurant visit & keep it on ice for a few days & use it for a meal.
We budget way ahead of time for big meals out while on vacation, and know that all meals are included on the ship, which is great!
Planning ahead of time for everything is really the key.