I tell them that I will sacrifice a goat to Thor. Just kidding.
I usually go with the "I'll keep you in my thoughts" line as well. I feel the same way as I do when someone sneezes. I know it is a custom to say something when someone sneezes or when someone is very ill, but I feel it is a pointless and silly.
Human beings are the most deceptive mammals to sprout off the evolutionary tree. Deception comes as natural to us as breathing. We use deception every day of our lives. To tell another person a lie that you know will bring comfort to that person is, in my opinion, the proper thing to do. There's no reason to feel guilty. Your not being a traitor to your personal view of reality. You are just a caring human being.
I don't have a goat to sacrifice to Thor, but, I have a coupla' chickens.
I say "I'm thinking about you/them" etc....and sometimes say "I'm sending my love & best wishes...."