The mind continually works when awake or asleep.
When I find my mind wondering during PR ... I imagine putting tangerine back and behind my head. Or, I try chanting Re-lax, Relax .. to get back on track.
It's also important to commit to a time limit of how long you are willing to devote to PR'ing for that session. If you can only do 15 minutes then say that you will give 100% concentration for that period of time.
You may wish to take some additional time in getting into state ... you can tell when you have entered the state when your mind and body are calm.
Remember, it's ok to be human and have thoughts come and go ... that's the trick, when they do come, gently let them go and concentrate on the task at hand ... PR'ing.
Humbily,
Michael Saikali