I've been using FreshBooks [1] for invoicing and tracking payments. It's a very easy to use online service, of which there are many, but they also have some nice newsletter and blog items that I enjoy - a "we know our customers because we are like our customers" mentality.

This article: 7 Ways I've Almost Killed Freshbooks [2] makes some good points about building a startup. I especially like this quote: "Doubt is born out of fatigue and loneliness, and there is a lot of both when you are running a start up. Hang in there and keep your feet moving - there’s still a lot of time for you to change the world."