When I have a trim problem or difficulty mastering some maneuver, I write it up in the log and the act of writing itself often triggers ideas for solutions. When I do solve something, that goes in the log too and it has happened that, at a later time, a similar problem arose and I found the log entry and a solution. When I make changes to the center of gravity or the needle valve, I might forget that a week later but the log can remind me. Changing the engine, propeller, receiver, frequency or any other equipment should be logged. If I had logged the propeller size on the P51D, I would not have had to remove the spinner cone to find out!