Practice helps for sure. When your mind wanders just bring it back gently... keeping the pace. It's unlikely you missed anything while photoreading if you managed to maintain the blip page and a steady pace. Sometimes thoughts come into my head about the book I'm photoreading I sort of just do a Spock and say 'interesting' and keep going, fall back into the chant... unless I am specifically searching for some information, then I pull up and do a superread and dip.
It doesn't really matter what you say for the chant... saying relax three times in a row might be your inner mind asking you to relax and go with the flow 
. Remember if your mind wanders while you are photoreading it may be giving you important information, if it doesn't seem relevant just tell yourself gently you'll think about it later.
Alex
