I prefer to keep it as simple as possible. Proving that God cannot possibly exist is easy enough but when they counter with some variant of "well then where DID the universe come from?" I just honestly answer, "I don't know." Christians preach that humility and honesty are virtues so they should, on some level, be able to respect such an answer. We can develop theories by referring to various bits of evidence that we find but we can never know such a thing for certain. Personally, I don't even care because the answer has no pratical bearing on me or anyone else whatsoever.