adding to this: if your issue is one of focus, try a pomodoro timer. it's a great way to shut off that voice nagging you about all the other things you need to remember to do, because you can forget about all of those and know that in 25 minutes you'll have an opportunity to reconsider them. if you're really strict with your time management, another approach is to use detailed calendars, but that's a lot more work than just a simple timer.
