There are many ways to contact past clients, but its just the importance of actually contacting them. Your quite simply leaving vast amounts of cash on the table by not contacting them. Too many people presume that after doing an amazing job that the client will definitely be contacting them.
This is probably true, however a lot of people simply forget who you are, and what you are called - and easily LOSE your business card/flyer. They resort to going in to the phone book or the internet looking for you (even though they have forgotten what your called) - and then give in and pick another company.
Thats the end of that client (and many others too). Same as Shaun, if you have been contacting clients every 6 months or every year etc etc and they havent responded - then I would cap it at that. 5yrs max for me.