Interruptions from calls, administrative overhead, self-employment tax, and health insurance costs.
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.
Keeping the pipeline full for when projects end or go on hiatus.
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.
I would add to that by saying taking advantage of slack time is difficult. Taking breaks, holidays, sabbaticals, is tough when the mind is used to thinking about getting more work in between projects.
Conversely, keeping the pipeline full while engaged on other projects. This applies especially if you are booking in one-month blocks. How do you keep incoming leads hot when you won't be available for at least 60 days?
I'm very upfront about my availability. I also let people know weeks in advance when I will be available. And in between that time I stay visible at my favorite events/meetups and in social media (publishing little open source things), asking questions, etc.
About a year ago I conducted a small-scale survey on how freelancers approach marketing and sales and what their biggest problems in these areas (and in general) are:
Sorry for that. Google Charts apparently can cause layout problems on mobile devices. The static image version linked in the article should work fine, though.
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.