For me, it was my mother-in-law. She planned, long ago to spend the weekend at my house. Her trip made the perfect cover for the party since she was going to be there, she could communicate with those who were planning on attending the party.
Nothing ruins a surprise like someone being at the house too early. Have the guests plan to meet somewhere else, could be as close by as a block over. Once everyone who is attending the party is there, drive over together. This way, all of the guests arrive at once.
I'll admit, it was quite odd to see all these cars pull into my driveway at once, I thought they were all lost!
Don't assume the guest of honor will have something you'll need on hand. Bring EVERYTHING. My party throwers brought balloons, table cloths, cups, napkins, plates, drinks and all of the food items they wanted. One minute my dinning room table was empty, the next it had a buffet of items on it. Very impressive.
To be fair, the guest of honor was not expecting a house full of people so once the party winds down, a few of the party throwers should hang back and clean up (at least the food they brought). I had no problem taking a broom to my floors but it was very nice of my party throwers to package up any food that wasn't eaten and either put it in my refrigerator, threw it away or took it back out of the house with them.
Thinking of throwing a surprise party for someone? Why not throw it for them in their own home? Just keep in mind the size of the guest of honor's house when determining the guest list and get to planning!