…and you’re not a believer, here are some appropriate responses from Peter Mosley at Patheos.
This comes from an earlier conversation I had with Dan Blinn, lawyer, humanist, and a hell of a good guy. My reaction to someone who says “I’ll pray for you” is to respond “And I shall pray for you,” and then I make a big show of getting their name so that I can pray for them personally. This is a completely non-Christian up-yours response that I should probably work on but don’t.
Dan’s suggestions — via this article — are better.