I am terrible at keeping numbers in my head...
I swim six days a week ACROSS the deep end at our local YMCA...it's kind of a meditative time and I sure don't want to have to remember how many times I've come across...but now that's I'm getting in better shape, I'm feeling just a little competitive -- with myself.
I've started keeping five rubber bands on my right wrist...each time I swim across & back I move a band to my left wrist. (I can keep the sets of 5 in my head :) So today I did 15 laps.
Hopefully I don't drop one and have to dive for the bottom searching for it.