The Spanish equivalent of "break a leg," as it turns out, is "¡Mucha mierda!" Literally, "A lot of shit!" According to this site, the expression comes from a time when horses were the common mode of transportation, and so a lot of horse excrement outside the theatre meant you had a lot of people come to see your play.
Tomorrow night is the dress rehearsal, then Friday is opening night. Wish me a lot of shit!