Some NonDirective methods make use of a mantra. The mantra is an excellent tool, but it is often misunderstood.
Some methods do not use a mantra.
It's misunderstood especially by those who are used to other forms of meditation or prayer, where the mantra actually means something. That's the kicker here, the mantra actually means nothing - and it does its work for you best that way.
In a NonDirective context, the mantra will give your mind something to lock onto that will provide a bit of distraction (from thought) so that you can transcend (move beyond) your thoughts, yet, it is not something that you should concentrate on too hard, or think about too much in itself. Thinking too hard about the mantra is still thinking, which is what we're trying to get past.
The mantra might even disappear, and you might be left if a very quiet and peaceful place.
If you choose a mantra that means too much to you, or provokes too much thought, you may have some pretty amazing realizations about that mantra (or rather - the thoughts behind it), but you will not transcend thought, because realizations are thought.
It's most important to remember to think about the mantra as a vehicle, not the goal, not the main focus. Our daily, "doing" mind will want to focus on it in daily, "doing" ways, since it's the only thing that seems to be going on in meditation.
You might think about changing it, or moving its "location" in your head. You might think about changing the speed, or pitch or timbre.
Remember that any of these things are thoughts of the "mind" that we're trying to heal with meditation, and therefore, they're probably thoughts we want to let go of.
Don't dress up your "vehicle", just ride it. Let the mantra remain the way it started out, just let it be. Eventually it might become more abstract anyway, shedding all of the "dressing up" you did.
Finally, any type of changing or dressing up of the mantra is just a distraction from your main goal. If a mantra is your vehicle - ride it, don't spend time worrying about what it looks like or sounds like. These things are distractions, and will more than likely cost you time that you could be spending in a better, more transcendent state.