Do all the difficult things in the morning. Don't overwhelm yourself with long hours, do things little by little. I spend 2 hours doing 1 hours work because I really need my downtime, it's not efficient but I don't want grey hairs by 30.
Diet is also key, have fruits to keep you going on your breaks, if you drink coffee/fizzy drinks or if you smoke, your productivity levels will plummet at some point in the day.