I started keeping a sailing log nine years ago...pretty basic. I list where I sailed, what gear I was on, who I sailed with, what technique I worked on etc. It's also great for making notes about gear maintenance, rigging notes etc. I can't tell you the number of times I have flipped through an old sailing diary and discovered some note to myself that I'd forgetten but needed to know!
-Michael (sailed 70 days last year)