"Empty your cup."
I first read this phrase in a book entitled Zen in the Martial Arts. This chapter of the book talks all about when one learns something they must first empty their mind of any past knowledge and idea. This doesn't just apply to martial arts, it applies to all of life. I often have the problem where I am trying to go to bed and my thoughts will drift to past events or conflicts. during these times I try to remind myself that those events are in the past. If they are something I absolutely must think about I try to remember that there is a time and place for that, and it is not when i should be sleeping.
This goes hand in hand another lesson from that book; to live in the moment. In the present there are no regrets as there can be in the past. If you fully devote yourself to whatever task you are trying to accomplish your experiences will be far more rich and fulfilling. Even sleeping will be more restful if one can let go of their daily trials.