At last a thread I can contribute to -- been lurking for about a year
I use a golf tee on the edge of the pool like the hour hand on a clock. Nudge it one number per lap 1 through 10 then go back to 1 and repeat. Cut a flat side on it so it doesn't roll around. So far, I have no trouble remembering my laps to the nearest ten.
I guess you can forget this method if you flip your turns!!!