I find that when I have trouble concentrating on things (be it poker, or anything else), I will take a break from whatever it is I can't concentrate on.
Step away, take some time for yourself and do something that you enjoy and doesn't take much thought process/effort or that you can pretty much do on auto-pilot.
For me, I find that when I need to let my mind rest, I will sit down with a coffee or tea and read a book (if its in the evenings or during the week) or go out for a ride on my motorbike if I have the time (usually on the weekends). I find that the latter really helps me unwind and relax, just riding with no particular destination in mind is always nice.