Acknowledge to the person the surprise is for that you are doing something for them … So that they don’t go off and plan their own thing in a fit of, “but what about meeee?!”
Don’t tell your friend who can’t keep a secret til the day before. If that friend is someone who is busy, invite him or her to some fictional activity that is the same time as the actual event.
First and foremost – remember that the party is for … well, whoever it’s for. It’s not for you. Plan for their tastes, not yours!
Don’t email the spouse of the person the party is for, in case the recipient of the surprise party happens to have access to the emails of the spouse (sure, this seems specific – but it happens)
Anywho, see you in a few days for your non surprise party, Mom.