It depends. Some people say it like "You're such an awful person, I'll pray for you" which I would guess is bad. But I take it you just mean in a good way. The only thing to say I think it would be "Thank you". For the same reason Ayn Rand said in an interview "thank God for America". But before saying it she was explaining that, of course, she didn't believed in God but she liked what that expression meant which is something like "being grateful for the highest possible reason". So applying the same thinking you could be grateful in the sense that, even though you don't share his/her believes, he/she will ask the "highest possible luck provider" to give you luck.. Or something like that.
answered Jan 22 '13 at 22:44
Juan Diego dAnconia