Cut back on things that guests won't notice. Carley, The Knot's editor, suggests eliminating a couple of courses from the dinner or only buying flowers that are in season.
Also, don't be afraid to put on the brakes. If a voice in the back of your head is starting to tell you the spending is getting out of control, listen carefully. You don't want to end up resenting the money you spend. Nor do you want to overindulge so much that you, the guest of honor, and all the participants forget the real meaning of the milestone you're celebrating.