Years ago my MIL told me that when her very smart and scientific-minded kids started asking if there was a santa claus, she told them that if there wasn't one, there would be no more presents under the tree from "SC". (There were gifts from Mom and Dad and gifts from SC - that helped with the understanding that people give presents in addition to Santa...) Needless to say, they stopped asking about the truth of Santa in order to keep getting SC gifts...
When my older son asked me, I said the same thing to him that my MIL said to my husband. Like his dad, he was smart enough not to push the point!
That being said, I saw the book "The Polar Express" and bought it (this was before the movie.) I bought a bell that looked like the one in the book and hung it on a hook on the frig. I read the book to them and ever so often when they were in the kitchen, I'd say to them "Can you hear the bell?" and I'd ring it for them. For years I read that book to them at Christmas time.
We still give my kids presents from "SC". They have never told me that they don't believe in Santa. Of course, as adults now, they don't believe in Santa anymore, but we enjoyed "hearing the bell" as a family for a long time!