15. Take care of your primary asset
The primary asset of your business is you. If you become burned out, or ill, or your skills get out of date, your business will suffer.
There are no sick days, there are rarely vacations, and deadlines don't care how you feel.
Be aware of new developments in your field, but spend R&D time wisely -- not only on things your clients want, but also on things that excite you.
Occasionally turn off the computer and go outside. Socialize with friends, spend quality time with family and loved ones, read a nontechnical book, and enjoy being human.
Most importantly, when the going gets rough, take time to recharge and remember why you started this journey. The payoff of time off to refocus is tremendous.