There's been a lot of good advice here but there is an extra tip that I have always found works wonders:

If you're having trouble shutting up that 'monkey mind' that jumps from one subject to the next then use distance. You know how on the top of a mountain all your problems seem so far away then use that. Mentally project yourself to a place far away from all your immediate problems and your mind will stop focusing on them so much.

This works very well and I have taken it to the extreme of projecting myself out into space well beyond our solar system. Out there all of mankinds problems are so infinitesimal insignificant let alone my own that I can completely just be one with myself. When you come back you will find that you feel incredibly peaceful and it's like somone has turned down the volume control in your brain so you can hear yourself again.