Do. not. do. a. trick.
Don't.. I'm serious.
Make it special.. as simple as taking her to a dinner and a movie, then telling her how awesome it would be to go with her.
I would also ask between 5 to 3 weeks before hand. Don't do it too far in advance. She might get sick of you and say JUST KIDDING I don't want to go with you... I've had a better offer.
In the mean time, learn to feel comfortable on the dance floor and practice some dance moves. You don't want to ruin her prom by being a wall flower, or "that guy" that does voluntary card tricks to everyone making her wait for you and watch you do the same thing over and over again. Trust me... it can only end bad if that happens.
That said. Have fun and be safe!
Love,
Katie