I don't know if you'd consider this homeopathic or not (probably not) but nicotine patches can help with concentration. You can even use the low dose patches and expose one quarter of them at a time. It works up to a point, but I unfortunately started to lose circulation in my hands. Burger's disease or something.

I've not had much luck with anything else. I've tried American ginseng with ginkgo biloba without much effect.