I find it particularly handy when I can't sleep if I'm worrying about something. For example say, I forgot to pick up my clothes from the dry cleaners, so I'd write it down and tell myself I've written it down so now I know I won't forget it can worry about it tomorrow and can let that thought go. It may not always work depending on your thoughts but the very action of writing it down and knowing that it can return to them later can be useful and give you peace of mind to at least clear one distracting thought.
Meditation can help as well, but it may require some initial practice along with some patience and dedication. Starting small say 5 minutes a day can help you get into the mode and habit with this (see Introduction to medtiation for tips on doing so).
Someone once told me too that if you are planning to start fresh on anything then it helps by not only telling yourself you're starting from anew but also by washing your hands too. That way you can feel that the old things can be washed away from these action. Our minds are interesting in the way that they work in that manner, so it is most likely the engagement of our senses that allows us to feel and remember our action point that way.