"Work smarter, not harder." Chances are, you've heard this phrase before -- and probably more than once. The philosophy behind working "smart" is to maximize your productivity when you are working so that you can get more stuff done in shorter periods of time.
The best one for you depends entirely on your working style and personal preferences.
You can use a physical notebook around everywhere you go, but it's easier to use a to-do list app or tool that syncs across all your devices. That way, you can access your to-do items whenever and wherever you need to, whether you're at your desk, in a meeting, or on a business trip.