Carrieann,
I'm with MasterJ... the more orders the merrier...
But along the lines of your wishes. I would send out the thank you's at the end of the month of their order, with a coupon code that is set up until the end of the following month (mail 04/01, expires 04/30). The do the same the following month with a different code.
They are so easy to set up it only takes a minute or two. You can o it without publicizing it, so other customers do not even know about it.
HTH