I sit down, if possible in the circumstances, and begin paying attention to my breath.
I pay attention to it the same way I would pay attention to the tip of the paintbrush as I am painting.
I do this for several seconds or minutes at a time, until I'm able to achieve a calm state.
Sometimes I do it counting the breaths, which may work well with your attention specifics.
Remember that you are experiencing evolved behaviors and states which are being triggered in unproductive ways.
You can change these triggers by first dissipating their effects. This is what breath meditation did for me, over the course of several years.
I also read that meditating in a quiet room with no distractions is sort of like the "5-pound weights" of meditation, where you begin to learn how to practice... But the real payoff is knowing how to meditate when you are in a stressful situation, when the techniques can really help. I read this in Wherever You Go...
Because I don't get many opportunities to go to hour-long guided meditations or to take a 14-day silent meditation retreat, etc., I decided that I would combine these two ideas, and jump straight to the "10-pounders".
I decided that whenever I was a) waiting for something b) annoyed by something, or c) at an overly long work meeting, I would seize on this wonderful opportunity to meditate and just start meditating, without changing anything about myself externally, on my breath.
Breathing, external sounds, and tactile sensation, are three things I've heard teachers suggest meditating on, one or two at a time. Breathing works well for me, but not for everyone.
When I am "meditating on [something]", it means I'm paying attention to it the way I pay attention to the tip of the pencil when I'm drawing, or the paintbrush when I paint, or the cursor area when I'm typing on screen. With breathing, I sometimes think about how each breath out is connected to the next breath in, all the way to the beginning... Sometimes I also think about how the breath connects the conscious with the automatic.
I've been doing this for several years now, and I've found it to be one of the most helpful things to help in almost any situation.
With regards to mental state, it also helps to think about that "this is temporary"