Single most Useful Thing that has Improved your Efficiency& Productivity?

Has to be a planer-diary for me, I write stuff down What I want to do the day before and than try to do every single thing from my list and crossing out the things I have already done. I'm thinking now about buying a digital watch which would help me keep better track of time with the help of timer/alarm. What's your one thing you wouldn't be without to help you be more productive?