When I was actively listing and selling real estate the most effective tool I ever used to stay in touch with clients and customers was a monthly email newsletter. I kept it simple and never wrote more than six things in a bullet type format. I found that this length worked well for me and was not too cumbersome for folks to actually read. I generally included links to new listings, sprinkled in community events, and sometimes shared insights. All in all I found this worked really well. In addition to this monthly shout out I also called former clients. I made it a habit to make five calls a day no matter what. Every once in awhile my timing was right on the money, but even if it wasn't I believe people were happy to know that I hadn't forgotten them.
Essentially people want to know that they are memorable. Seldom do folks spend more money than they will for a home, and while the process of doing so is finite, the impact of this major transaction will last a lifetime. Accordingly, if you let people know you remember them, value their business, and appreciate their support your efforts will be acknowledged.
I have talked before about client appreciation parties which I did and were remembered for awhile. However, in my view, the mainstay of any relationship is ongoing connection. Doing a monthly email newsletter is easy and cost effective, but the biggest bonus is to make the calls or stop by every so often, so that former and potential clients know that they are more than a paycheck to you, but someone who matters and is memorable.