The way I do it is quite simple but works for me.
1. I create a CUSTOMERS folder
2. I create subfolders such as ACTIVE CLIENTS, DELIVERED, PENDING PAYMENT, etc.
3. I create a project for each existing client and place it in its corresponding folder.
4. In the client's memo, I capture phone, email, address, contact info, etc. In short, as much information as possible so I can have it handy. I also write a short recap of the client's personality, line of business, etc.
5. Every time I call a client or send an email or receive an email, I enter a date in the client's memo and write a quick reference to the communique (ie. "Sent her email re pictures" or "called her about colors") That way I keep a dated log of all activities. Note: now that I have a Mac I just drag and drop the email and it creates a link for future reference but the way I described is for iPhone/iPod alone.
6. Every task I must perform, I enter within the customer's project as usual and just prioritize it, context it or date it depending on the system I use.
7. When a client has ben delivered and owes money, I move the entire project to another folder. In this case, it would be moved to PENDING PAYMENT. You can create as many folders as needed buy I try to keep it to the bare minimum.
The work method I use is a combination of GTD and Franklin Covey. I create a context for phone calls, another one for emails, one for NEXT. I do not use contexts for places since I am pretty much at the same place every day.
It works for me and there is permanent record of all my relationship with the client form the original approach all the way to the final delivery and payment. Some of my clients are under a maintenance plan and it also helps me to bill them and show them what actions have been performed.
Let me know if you have any doubts and if it worked for you.