Another BigWaveDave'r here....I use the custom log they programmed. It is fun to look back over the season and remember the sessions. It allows some simple stats of location and gear used too -- good for resale, can tell exactly how many sessions used, etc. Also is interesting to be able to look up things like if this 2 week no-wind spell is typical for October or unusual.
But prior to my 2 years kiting, I've been keeping a surfing log on a calendar for 15 years.....I have a stack of paper calendars with little scrawled notes. I do this to try and preserve some memory, so it doesn't become a blur from year to year to year. Make sure you note who you were with and anything memorable that happened, as years from now, that's way more important than wave conditions on just another typical day.
On a similar note, I have hours of video footage of me and my friends learning to surf 20 years ago. Unbelievably lame and boring....interspersed is an image once in a while of us when we were young....wish I'd filmed more of that, as it is way more interesting. Everyone hates being filmed, but it is gold later!