get a fast paced job that requires thinking on your feet :P when I am at work on a buisy day I dont have time for any other thought besides the ones that include what I need to be doing at work. It is so blissful to have a mind free of personal worries for a few hours.
Get involved in some work
Breathing exercises always help to distract your mind from wondering...
It's not a button you can press on it to turn it off, you have to practice how to stop extensive thinking by distracting it, with the time you will be better
You can not avoid to think, and it never is too much. only, not be worried by your toughts as that are you.
Get interested in a good book.