Any tips for throwing a surprise party?

Since the element of surprise is the most important part of a surprise party, take care to check that your party decorations aren’t at all visible from outside the venue. If you’re using strobes or any special lighting, make sure the switches are easily accessible so that the flashes and colors won’t be a giveaway.
Fill the party room with decorations that will truly shock the guest of honor: Cover the ceiling with helium balloons, drape streamers from fixtures, and give guests champagne poppers and silly string to release when the guest of honor enters.