Various people advocate for using MITs—Most Important Tasks—in your planning. Though the specifics change, the MIT theory is generally the same—that is, pick only the most important tasks to put on your to do list, and keep the list short. A common rule is to pick a max. of three MITs each day.
I totally advocate the use of MITs. My best trick is to just shut the lid of my MacBook and do most of my stuff on pen and paper, then key it into the computer later. It sounds like I’m going back to the stone age, but it works wonders for me.
Belle Beth Cooper has a great article on productivity vs getting things done.