"Buona Fortuna!" otherwise you can use the very common "In Bocca al lupo" and the reply is "crepi!" that literaly is "in the wolf's mouth" and the reply used "die! (the poor wolf)" more vulgar (not so polite), but in some young & goliardic context is used so pay attention ;) "In culo alla balena" and the reply is "speriamo non caghi" (trans. "in the whale's ass" "i hope she won't s**ts) Ciao!

Buona fortuna