A client you already have is a thousand times easier to work with and get more work from, than trying to find a new one. This is a solid business fact that many people overlook. They are too busy chasing that new lead to remember the consistent leads they already have!
Working with the same client who you enjoy working with makes photography much more fun. Over the years I have developed some great business relationships that lead me to more and more work. These things did not happen overnight however. Gaining a client's trust, and becomind the "go-to" guy usually takes more than just one photo shoot, so how do you get more shoots?
Contact the client! By sending out regular e-mails to clients they begin to take more notice of my work, and think of me first for an upcoming shoot. There is a definite art to these e-mails that makes all the difference.
Typically, jumping to the point is the most effective method. Spending too much time writing about what you are up to and asking general questions can make the e-mail less interesting. Sharing a link to a recent shoot however can work out great if it relates to what they need and is not for a competing company. I usually will ask specifically if they have any current or upcoming photo needs. This lets them know that you are still in the business, and interested in shooting more. It might even get them thinking about some project that they've put on the back burner, but could get started right away with a photographer.
How often you send these really depends on the client. Too many e-mails and they are going to be annoyed and you will lower your chances, but sending one a month or so can prove to be very effective. There are many times when simple e-mails like this have landed me more work.
Photography is an art first, but to make a living at it is also a business. By being business savvy, you can land more work and make a living as a photographer! Start sending those e-mails and crafting successful business relationships!
A great resource that will help you with the business side of things is The Great Business Project. There is also a forum on the site where you can ask questions. It's not photography specific, but really gets the thought process rolling and will motivate you to get out there and make things happen!