Here are the steps to follow if you ever want to ruin someone else's surprise birthday party. It doesn't matter whose party it is (in this case, it was my sister-in-law's 30th bday)...if you follow this guide, you'll be sure to ruin it for yourself and most of those in attendance:
Step One: Make sure you bring an 18-month old child with you. This is clutch.
Step Two: Even though the b'day party planner (my brother) has carefully laid out what he expects of everyone, don't pay any attention to his guidelines. One way of doing this is to tell everyone that it would be funnier if we hid somewhere else (say, in the bedroom rather than in the living room as originally planned). This way, you'll not only surprise the birthday girl, but you'll be able to pull one over on the party planner.
Step Three: Although you carefully hide everyone's vehicles down the street and hide their shoes and purses on the back deck, make sure you leave up the party decorations and leave the appetizers on the counter. This way, the birthday girl will be suspecting something long before anyone shouts, "Surprise!".
Step Four: This is where the 18-month old gets to do her part. Make sure the "surprise" room is full of adults (most of whom have had children themselves) who expect the child to stay completely quiet. After all, she's been on this planet for a year and a half - she should know all the ins and outs of what's involved with a surprise party and be able to keep her trap shut. For good measure, throw in a grandmother who likes to say, "I bet Kat ruins the surprise" three or four times in a row.
Step Five: As the birthday girl is coming down the hall, have the 18-month old call out, "Hello?" and completely blow everything. Afterwards, you can berate her for doing exactly what you've been teaching her for the past six months. This should thoroughly screw her up. Also, once everyone yells "Surprise!", listen quietly for the grandmother to say, "I knew Kat would ruin it". You may want to bite your lip at this point, to stop yourself from crying.
Step Six: Ten minutes after the party has started, give the child a helium balloon to run around with. Then let her fall, face first, on the floor so that she can put her teeth through her upper lip. This way, you get to momentarily leave the party to take her to a walk-in clinic (so you don't have to listen to the grandmother tell everyone else how the child ruined the surprise). You'll likely discover that your daughter is just fine and does not require stitches, but the doctor will offer you a Valium to calm down.