Keeping in touch is crucial not only with past clients but with your entire database. I suggest you research and invest in one of the datatbase systems with all the bells and whistles. I use TopProducer because I am with Keller Williams and we have a version with our marketing tools on there.
How it works is that while you are entering a client into the address book you will be soucing them and putting them into contact catagories ( Buyer, Seller, Friend etc..) You will also put them onto an action plan that will remind you each time you are supposed to contact them and what type of activity to use(call, letter, e-mail etc..) They have pre written material or you can create your own.
Other than the contacts you have the ability to create and save your own marketing flyers that can be published to the web and used as a link for your website, craigslist etc.. Instantly create a community report simply by plugging in the zip code. Mass E-mail, lead capture, Buyer and Listing presentations and honestly a lot more.
I have never tried any of the other popular systems but look into them and find the one that works best for you and it will keep you in the minds of all of your contacts.