Having a suite of unit tests I can rely on. It helps me avoid the paralysis that occurs when I wonder "what else might have been touched by this code I changed?

Having a suite of unit tests I can rely on. It helps me avoid the paralysis that occurs when I wonder "what else might have been touched by this code I changed?" I can give that ? a couple minutes thought, write whatever changes I need, and then run the suite to see whether I missed anything. That factor more than almost anything else helps me write code quickly.