How do you politely refuse someones offer to pray?

Do not go into lengthy explanations, just excuse yourself and go elsewhere. You do not have to explain anything and it would be better if you didn't try. I have a friend who, despite my requests not to, sends me chain letters about prayer (I am Christian and do pray but do not feel like participating in chain emails on God's behalf, IMO it cheapens religion) so now I just delete them.