After you breathe in, there is a moment where you aren't breathing at all, before your breath turns outward again. Start off by breathing normally, without putting in any extra effort. And slowly make an effort to focus on this moment, between two breaths.
This is known as Anapanasati Yoga. Without wishing to get too meta on the first post, this is a technique that was supposedly used by the Buddha himself, to achieve enlightenment.
My Experience with this Technique
What I liked about this most, was the fact that I could switch into this technique for a quick second, while I was in the middle of something else. It can be done in an instant. However, when I tried to sit down and just focus on the gap between two breaths for a full half an hour, I found it hard to keep concentration, harder than when I try other meditation techniques.