I owe Bill Herzog money, so I have to keep plugging his books (just kidding) but on p. 59 of 'Spoon Fishing for Steelhead' (Amato books) he lists what goes into his journal. Nothing particularly special except he places particular emphasis on water temp (omits barometric pressure though -- seems an oversight).

I keep more of a narrative when I'm on those week-long expeditions to one place or another. Aside from water temp, viz and height I spend more time recounting fish caught and lost, as well as interesting things seen along the way. I used to bet rather heavily on fishing -- first fish of the day, most fish, most species and big fish -- for both the day and cumulative over the trip, so there a lot of Bingo card tables in my notes, too. (I still remember one guide in the salt telling us he never used a net for silvers. I kept saying 'This is a very valuable fish. Please net it!' Of course, the leader broke as he hoisted the modest 11-12lb silver aboard and the fish swam free. My 5 buddies laughed their butts off knowing that we'd bet an outrageous amount on who would be the first person to catch a fish on that particular trip. Within minutes, my wallet was lighter -- and the captain used a net to add insult to injury!)

When I get back home, I often type up the notes and then bore the bejesus out of my fishing friends by Emailing them the full trip report. But there's nothing like pulling out the write-up of trip to Alaska in '94 and being able to recall the good, the bad and the ugly like it was on video tape simply because you had a couple of paragraphs on each day's fishing. Definitely worth doing.