A better question is, how do you convince somebody who is trying to convince someone that he can't convince somebody?
Anyway, the best way to convince somebody is via facts and proofs, as well as anecdotal and experiential evidence to back it up (or be a silver tongued cult leader, but let's not go into that). God in the Abrahamic sense often seems to be transcendental of physical reality (hence why nobody can seem to give definitive proof either way), so giving facts and proofs is naturally ruled out.
So that leaves anecdotal and experiential evidence, which leaves you with a hopeless cause. Deists and Charismatics are convinced of their religion because of these evidences, and you would have to override a lot of conditioning to effect a change.
That is assuming they are a reasonable individual. As one female named Elizabeth Gaskell once said (probably), "I'll not listen to reason... reason always means what someone else has got to say."
*shakes eight ball* Outlook not so good.