Divide and Conquer is a strategy you might've heard of in the context of politics or war. It can also be used in less nefarious ways, to improve your own concentration.
Sit comfortably, close your eyes and begin by focussing on the breath. Make a quiet mental note to yourself of the word breath, on each in breath and out breath. When a thought arises, the usual advice is to let it go and return to the breath. In this meditation, when your mind starts to wander, just make the mental note of: not-breath, before letting that thought go. For the next 10-20 minutes as you do this meditation, characterise everything in this way. As either a breath or a non-breath.
A key to this meditation is to avoid feeling frustrated if you find yourself saying "non-breath" over and over in your mind. I certainly did, as my mind was still settling down. Remember that the aim is not to stop all of these thoughts, all you have to do is recognise that they're not thoughts about the breath, and gently let them go when they do arise.
Source: Real Happiness - Sharon Salzberg