Knowing how to decline projects

This is one of the traps that took me the longest to avoid, partly because when you're starting out, it's not necessarily easy to turn down work, especially for financial reasons, but it can be very beneficial.

There's a pseudo-statistical law called the "Pareto" law that applies to many things in life. When we apply it to being an independent developer, the law tells us that about 20% of your clients generate 80% of your revenue, and that the remaining 80% also generate 80% of your mental load.