I haven't really got a good enough grasp on it myself to teach anyone else, but you could start by reading The Power of Now by Eckhart Tolle if you haven't already. Probably meditation of some sort would be helpful too. I don't really know a lot about different kinds of meditation, but maybe someone else here will have some more answers for you.
Basically, whenever you notice yourself thinking too much, mulling over the past or worrying about the future, just observe your thoughts. Realize that you are thinking. Don't analyze anything or try to figure out the answers. Then focus on the present moment. The past and future are illusions. This moment is all there is.
If you read the book I suggested it will probably make a lot more sense than my ramblings!
