The best way to do this is to rely on a customer relations management system (CRM). Shared spreadsheets can get you started but you can rapidly run out of steam even in the first year - even if you are a solo-preneur. The complexity starts to explode after you get beyond 50 contacts.
There are high end CRM systems and there are free ones. Zoho CRM provides a free solution that works to first level of approximation. You can perform most basic tasks such as track your communications with a given contact, keep track of your prospects and run reports on closed sales and other sales metrics, provided you are ok with dealing with a rather spartan user experience.
You can always spring for salesforce.com when you are big enough.