One way to do this is by choosing 1–3 Most Important Tasks (MITs). These are the big, tough tasks for your day that you really need to get done. The ones that will keep you in the office past finishing time or working after dinner if you don’t get through them. Leo Babauta advocates doing these before you move on to other tasks:
“Do your MITs first thing in the morning, either at home or when you first get to work. If you put them off to later, you will get busy and run out of time to do them. Get them out of the way, and the rest of the day is gravy!”