There is a little device you can buy that attaches to the side of the pool, under the water and you hit it every time you get to that end. I think its about thirty quid as I was thinking of buying one myself as I have exactly the same problem. So busy thinking 'breathe out, rotate body, stretch forward in glide, kick legs' etc etc that I completely forget how many laps I've done!
i count it in my strokes before breathing! 1,2,3 then 2,2,3 3,2,3 etc. Tis ok as long as you dont get a hair in your mouth (when you have no hair) and have to stop at one end like a cat with a hair ball!!
I received my sport count last week and it works a treat no more worrying about how many legnths I have completed and I get to view my splits for each legnth afterwards. Two downsides I can see ... one been the drag factor of actually wearing it and, two been the time it takes to press the button - am sure its slowed me down lol.
Comments
The consensus seems to be a digital lapcounter that fits on your thumb.
Pretty nifty piece of kit and haven't seen a bad review
Here - http://www.activeplanet.co.uk/scindex.php