NEVER blow on it, it corrodes it, and destroys it.
Couch has got the right idea, though when I clean my cartridges I just use rubbing alcohol, without the water. I've heard a few sources tell me this is the way to go, though I can't remember any of them
It works like a dream on my Genesis and SNES carts. I don't know if it's better or not to mix it with water, but meh, it's working for me. I haven't had my SNES for too long, so I'm not sure about long-term damage >_>