Charge for the problem, not the hours. Hours are what it costs you, not what it is worth to them.
Write the scope down, including what is not included. Every painful project I have had started with a cheerful conversation and no document.
Late payment is normal, so plan for it. Keep enough runway that a slow client is an annoyance and not an emergency.
The best work comes from repeat clients who already trust you, which means the relationship is the asset, not the portfolio.
Placeholder essay. The real ones will be longer, with the specifics that make them worth reading.