Probably one of the most common means of centering is the mantra or a simple word or short phrase that holds a deep meaning for you. It acts as direct link to that sacred, calming space within you that is the goal of meditation – correctly chosen, the mantra should resonate that state.
In choosing a mantra, go with your gut reaction – the one that really sings to you. Like many of the answers we seek, when we are open to it – it presents itself immediately, selecting a mantra is no different. Depending on your religious background (if any) a mantra can relate back to that, for example: "Abba" is a common Christian mantra (it’s widely considered that it was used by Jesus) but then, it was also used by Ghandi as well (his mantra was his dying words). "OM" is probably the most common (and very sadly, the most stereotyped) example of a mantra. Found practically everywhere, this literally timeless mantra is the resonating tone of the third eye or 6th chakra. Personally speaking, I’ve found it to be probably the most powerful and harmonic mantra for use in meditation.
Basically, a mantra can be anything - as long as it is sacred to you. Generally speaking, it should be very short, no more than handful of words (example: "Om mani padme hum" which translates loosely from sanskrit as "lotus at the center of the garden"). Again, choose one that you feel comfortable with, look in your favorite sacred text, go into a few minutes of quiet meditation and see what comes to you… whatever you choose, let it be your own.
When meditating with a mantra, simply sit in the quiet in the meditative position you are comfortable with, letting the mantra flow with your breath within your mind. As you inhale, the only thought that you let through your mind is your mantra, nothing more. Using "OM" as an example – breathe in, "ooooooooooooooommmmmmmmmmmmm" a slight pause, holding it briefly, then exhale - again, the mantra is carried the length of your breath. And from there the cycle begins again: inhale with the mantra, hold for a moment of silence, exhale with the mantra. With each in and exhale you are drawn deeper inside your meditation, towards the center of yourself where there is true calm and nothing more. If random thoughts start to drift through your mind (and they always do!) don't get frustrated, smoothly bring back your mantra, clearing those other thoughts away and continue on.
Very simple and it will help deepen your meditation immensely. When first starting out in meditation, it's a common misconception that you should jump immediately into sitting in the pure silence with nothing else – while this probably works well for some and generally leads to frustration at not being able to focus. In this regard, using a mantra is excellent – it's widely regarded as one of the very best methods of centering yourself not only in meditation but within your "normal" day to day life as well (as is discussed in the previous lecture on breathing).