There are a number of methods and means of communication that you may choose from for the purpose. Emails, message boards, telephone, etc. are some common methods.

Email: while doing the project you must keep your client updated of the progress be sending emails. Even when you have finished with the project long time back, just keep sending him mails suggesting new ideas and other relevant things. This will make you remembered by him.
Have an occasional chat with him/them. Discuss business and other suggestions. You may also send your site address and updates of it.

Telephone: An occasional call will keep you in touch with your clients. You may do so in case of some urgent need of clarification during the project.
Visits: If your client is nearby your area, you should pay some personal visits. This will strengthen the reliability and your approach to him.

Message boards: If you have won a project through some freelance agency, communicating through message boards will be a safer mode to avoid any future controversies.