A mistake too many salespeople make is not keeping in touch with former clients. It's not uncommon for past clients to come to a point where they need your product or service again but don't remember how to get in touch with you. They are more likely to have your competitors' information handy. (Your competitors are still calling on your client even though you are not).
The odds of obtaining business from a former client are typically better than the odds of obtaining business from cold prospecting. So, keeping in touch with former clients is not only the professional thing to do, it also makes good business sense.
"Keeping in touch" doesn't mean pestering them - pushing for a sale. It simply means letting them know that you are still there, ready to provide service when necessary. This can be accomplished in various ways: a regularly scheduled phone call - just to say "hello;" a monthly or quarterly newsletter about industry events and trends; or a monthly e-mail regarding new products or services. Don't try to overwhelm your client; just make it easy for them to find you.