Build a Mailing List and Use it
This is something that I have never found myself doing well in the "real world" or On-Line, but if you can build a list of those that have shown interest in your products, send them a mailing once in awhile and let them know you are alive and what you have to offer. But do Not Spam them and if asked to be removed from your list make sure you do so ASAP.
Even though I do not use it aggressively, it has worked really well.
I have a large list and occasionally send to it, my list is built from those that ask to be put on our mailing list direct through email and through Guestbook scripts.
The Best program I have found to encourage new additions to my list is called SendMoreInfo which pays you and your client to receive Email. Their program is Spam Free 100% OptIn (optin meaning the person added to the list asked to be put on that list)
Another great program which is 100% Opt-In based is offered by the Mardox Refferal program and they even offer Target Marketing !