"Grazie della tua rapida risposta."

Translation: Thank you for your quick answer.

[crazy4hazy]

Is 'per la tua risposta rapida' ok? Grazie.

[marziotta]

I would rather use "della" than "per la", but sometimes we use "per" as well.

"Risposta rapida" is fine, but "rapida risposta" is more natural. Here http://italian.about.com/library/fare/blfare129a.htm
something on the topic, but "rapido" is not on the list. Sorry!

You can remember a short sentence to remember that "DI". "Grazie di tutto", "Thanks for everything", that is pretty useful, especially if you go to Italy and somebody who doesn't speak a word of English bring you to where you need to go because he cannot find a way to explain you how to do it. :D

[crazy4hazy]

Thank you, yes, it helps.