If at all possible, bill out jobs instead of hours. If you absolutely have to bill per-unit-time, choose as large a unit as you can get away with. Per-hour may work for attorneys, but when you have to watch the clock on every little thing as a developer, everyone loses.
Unless you are old, do whatever you can do to minimize self-employment tax. It's currently over 15%, and it's for a retirement system most of us will never get a dime out of.
If you have trustworthy people you can lean on for things like advertising, accounting, sales, etc., hold on to them for dear life and pay them fairly. The more you are able to double-down on your strengths, the better.
The dumbest mistake I've made has been this.
That slack time in between paying gigs can really kill financial momentum. Also, If I'm idle for more than two weeks that wastes not only money but also mental bandwidth. I go from thinking about work to thinking about how to get work, and that is stressful.
Always be networking. Always take meetings even if you are fully booked. Stay visible.
https://bjoernkw.com/2016/01/15/survey-for-it-freelancers-ho...