The key to hosting a surprise party is keeping it a surprise until the moment the guest of honor walks through the door. This can be very difficult as the various elements of planning a party are often obvious and hard to hide. Here are some tips:
First, if they want a party, allow them to plan one of their own, and simply plan your surprise before it. This way they expect to have a party, but on the date of it, they are very surprised. If they send out invites, etc. just make sure you inform the people they are unaware of what is really going on, and to disregard what they say or do.
Second, find ways to be sneaky with the invites. Written invites can be a problem for several reasons. They may get returned in the mail, they may be seen before they get mailed out, and getting all the contact info can be difficult. For surprise parties, it is often better to make a phone call, send a private message via Facebook, send out an email, or something else that is similar. If you do not know who to invite, a text sent from their phone to their whole contact list, asking people not to respond, letting them know it is a surprise is often effective, just be sure to erase the message from their sent box, and don’t get caught with their phone. Another effective measure is to turn the inviting over to someone who would know everyone they know, or have easy access to the information. Such as a spouse with their email password, or a friend with the same friend set, etc.
The next trick is getting the right amount of food. In most cases, with surprise parties you do not ask for an RSVP because often the host lives with the guest of honor. So, if this is the case for you, an effective thing to do is buy foods, treats, etc. that do not have to be used right away. While fresh fruits and veggies are nice, if they do not get used, they are wasted. Items you can freeze, or store for later (such as skittles or starbursts, candy bars, etc.) work well.
Last but not least, in order to make a surprise party effective, you have to get the cooperation of the guests. This means make sure they park out of sight so that the person the party is for does not recognize their cars. Make sure they arrive early so that they are not walking in at the same time, or after the guest of honor. It is not much of a surprise if there is no one there to yell “surprise” as they walk through the door. The best way to do this is to have guests arrive 30 minutes before the guest of honor, and collaborate with someone to not get them there until later, while keeping it a surprise.