"Remember that you cannot get eight billable hours out of an eight hour day."

This is an extremely important point. When I was freelancing, I was surprised by just how much non-billable overhead time I had (at least 5-10 hours a week when working with 3 clients). To reduce overhead, work with as few clients as possible. But you'll run into dead time that way, since jobs don't start on a dime.

I know a guy who has freelanced for 5 years and kept meticulous records. He knows what he's doing, but says every year he tops out at about 60% of his time being billable. Make sure you factor that into your rate.

It's really easy to take your rate and multiply by 40, thinking that's your weekly salary. This is pretty much never the case.

Also, don't forget taxes.