Organizing your client details is an integral part of your work if you work as a freelancer. Your client details are your most precious assets (aside from the skill or service your provide). If you work hard and provide a better service to your clients then there is a chance that as time passes, most of your work originates from those clients who have already worked with you.
In order to get ongoing work from your clients you need to keep in touch with them because, to be frank, no matter how important your service seems to you, for your client, you could easily be one of the hundreds of individuals he or she must be dealing with in a year, and the next time he or she needs the service you provide, he or she may end up working with someone else because he or she simply may have forgotten about you, or lost your contact details. Keeping in touch helps them contact you in the hour of need.
There is another reason you should organize your client details efficiently. If your work on multiple projects and you need to contact multiple clients multiple times, there should be no scope for confusion, losing contact details, or sending wrong information to wrong clients. It can cost you your work sometime.
This blog post reviews a few contact data management services (some free, some commercial) that can help you organize your client details efficiently.