Just a quick post to answer Mich's question:
One thing that helps me count laps is that I always start on the same side of the pool, so there's an "even" side and an "odd" side. I also generally do groups of sets -- I used to do my workout as a series of sets of 4, but now I do them as a series of sets of 10, mostly because the 10s are easier for me to remember. I also do think of the numbers when I'm swimming. So as I'm swimming I think "1" as I'm doing the first lap, etc. I guess this answers Watching and Weighting's question too -- I count up, not down. I have gotten into the habit of doing a couple of kickboard laps as a break between sets, which also helps sort of set the number I'm on in my head. Yesterday, I did two kick laps after the first 10, two after the second 10, one after my set of 30, two more after the next 10, and then four after my last 10 (because I didn't want to end on a weird number like 76). I don't really count the kick sets as part of my mile because I do them at a low level of effort.
I do get confused sometimes, but not that often. Yesterday I wasn't sure if I was on 33 or 35 (as I was heading down to the odd side of the pool) so I just counted it as 33.
There are lap counting gizmos but I've never researched them. I don't know how -- or if -- they work or how much they cost. The ones I've happened to see in magazines always seemed expensive. I figure it's not that important to me. I used to get more confused when swimming was harder for me, because I was focusing so much on technical stuff that I had less brainpower left for counting.
Eventually I should really lengthen these out and mix things up. I've been doing all freestyle but supposedly doing the other strokes is good for balance and endurance. I have a decent backstroke and breaststroke, but my butterfly looks like I'm drowning in a washing machine.