In order to budget properly, you've got to be prepared for all expenses. Because you're on vacation, it's important to indulge more than you normally would, so don't deny yourself. But if you aren't expecting those extra costs, they could put you in the poorhouse. Consider package deals. They're less flexible but usually save you money.
Cruises are especially good at getting the damage done up-front.
No matter what deal you get, there's almost always going to be extra costs that need to be considered. Know what you can afford. Your needs may be fulfilled without an elaborate trip. And don't forget, some of the best short vacations are often spent visiting friends or relatives who offer free room and board, a new environment and the chance to catch up. Some of us need to just up and leave our lives for a few months.