Mike
Yes I keep a fishing diary. Usually record date(!), venue, times fished, river height, wind strength and direction, flies hatching and at what time, size of fish caught (if over 2lbs or 17"), widlife seen etc. It is not exhaustive and if I have a poor day I sometimes can't be bothered, though these are often the days you learn most. PP keeps a far more detailed diary and writes it up without fail every day he goes.
I find it very useful. We have taken detailed notes on wind direction when on Don and frequently will choose a beat depending on which way the wind is blowing. Certain beats are sheltered from different directions. A bit anal perhaps but we try and stack as many things in our favour as possible and then leave the rest to Mother Nature.
Lets hope Don fishes better this Spring than last year which was the worst we have encountered, due to lack of rain. Roll on the new season!
Richard