One of our SunnyHillsMeditation.com members mentioned he has a "wandering mind" when it comes to using a mantra.
Seems to me we all have wandering minds.
I think the idea of a mantra is to practice it a few minutes at a time, until we start building up more focus.
The easiest way to focus, is to sit in easy pose, and meditate with a straight spine, eyes closed, and focusing at the brow point.
Also, tying the mantra to the breath, of course.
It's more challenging (= more difficult) to breathe with a mantra when we are moving around during the day.
Although, going on a walk, or going for a run, can put us in sort of a meditative trance that helps us focus on our breath and on our mantra.
My challenge is to try to breathe 4 times/minute (when I'm restful) with a mantra going, trying to do it all day long, or even for an hour, or even for 5 minutes or so.
If I'm more active, walking or working with body movement, then 8x/min is more reasonable.
The yogis say if we can breathe...