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. Hubspot has a free CRM tier that can be very helpful in the early days of a startup (and it may well be all that some small businesses will ever need, as it has reasonable feature sets for contact management, pipeline tracking and more.
Zoho CRM also 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.