Don’t use your head for mental storage, it wasn’t made for that purpose. David Allen says “Use your head to have ideas not to hold them”. In order to be able to focus it is essential not to be carrying around your tasks and responsibilities in your head. Not only does it cloud your thinking, but you risk missing appointments or forgetting to do things that need to be done. Get into the daily habit of getting everything out of your head. When you have it down on paper you can organize it into your system; appointments to your calendar and tasks to your task list. Implementing this habit will reduce your stress and increase your efficiency in dealing with current tasks.
From Stepcase Lifehack.