The day I went around a course saying to myself "concentrate, concentrate," I orienteered quite badly. You need to ask yourself questions, which I imagine you were doing as well, such as "am I going the right direction, what way does the hill slope, what should I see near the control, what should I see on either side of me, etc." Get the mind working, but forget the mantra :-)
Gail, you are correct, just saying "concentrate" isn't enough. When I found my mind wandering, however, it did help to bring me back to the moment. I was also saying things such as "look for the stream bending north" etc. But I have recently noticed that I need to get more specific at this, and identify more expected features in the circle. Very good point about getting the mind working. Something I will try at the next practice.
In the couple of really good races over the years where almost everything just goes right the one thing that I noticed they had in common was that about half way through I start talking to myself out loud about what I was expecting to see. I think it kept focused and kept me pushing. For some reason I've never tried to start a race that way but when things are flowing it happens naturally. I suppose I should try to train myself to do it more deliberately.