Great thread!
But I don't think we need to fill be limited by ONE thing. "One thing" - is a question of micro and macro. In macro retrospective there is nothing to tell expect that your dive is exactly that you do it. Yea, as told before. But in micro, each dive I learn something else. And not at the dive time only. I look around me at the land time and learning from the people around me. And this knowledge makes my diving safer. Let's go, share your experience.
So, what about me? Some most significant tips I learned...
It was at rescue course. We dived in the pool and one of students wasn't able to fill his BC with an air. Turned up, that inflator strips became weak and didn't connect inflator to BC tightly. So, all air was done out form BC. The good news it was a pool dive...
So, the tip I learned - look at your gear and service it regularly in general and specific about BC each time before diving session I connect my BC to the tank and regulator and fill it with an air completely in the washing pool to check possible leaks in the system.
Each time coming back after some break I take a minute at the first dive for practicing and refreshing of my skills about the mask and regulators. I found it more handy to practice it an 3-4 meters then at 25-30 meters...
Starting each dive look at my compass and notice my direction. Backward will let me home.