I only tend to lose concentration when returning, because I'm not playing at my own rhythm. So I have a couple of things that I do.
I do the same as you with my footwork, especially really concentrating on my weight going forward through the shot - for some reason this helps. I've found thinking about my swing (i.e. racquet speed) has the opposite effect as I second guess myself and either start to under hit or overhit.
It's kind of hard to describe but another thing that sometimes works for me is to get into position for the return before the opponent is ready to serve and just stand still and wait. Jumping up and down or turning my back and walking around, etc. just distracts me. Somehow this makes me feel that things are on my own terms and I have nothing else to do but concentrate on the coming serve.
