Make sure you get the facts. It reminds me of a Gierach story about fishing journals. We tend to get into trying to describe beauty and how we feel about the day, and that never comes across as well in writing and its never what you're looking for when you go back and read it in 5 years. Yeah, our reason for fishing in the first place is the beauty, exercise, and overall experience. But those are not your reasons for reading an old fishing journal! Facts.
Even among "facts", there is a sliding scale of objectivity. The more objective, the better, because your definition of "high" or "warm" will change with time.
For instance, if the stream has a gauge, write 200 cfs instead of "slightly high." Instead of saying "slightly off color", put it on a scale of 1-10, or better yet, construct a color card, place it in the front of the journal, and match the color. Water temperatures at several times throughout the day, and in several locations, are good notes to have. Don't say its a warm muggy day, say the high was 86, and it was 76 at dusk, with humidity at 60%.
Date, time, weather and water conditions, and hatches are probably most important, because more than likely, you'll be using it to try to predict when those hatches will occur. Noting the fish response is useful (they rose, they didn't rise, they were actively chasing nymphs under water, they wanted emergers, the small ones rose while the big ones chased nymphs, etc.).
How many you actually hooked and landed and what exact pattern you got em on is secondary and less important, it could even be negative. If you nymph all day, have 2 hits, and land them both, well in reality, thats a sucky day and a sucky situation, you were lucky to get what you got and it isn't something your future self wants to repeat. But if you had 50+ refusals, 25 misses, lost 3, and landed 2, well the opportunity is there for one heck of a day. You were either not good enough, or just unlucky (we all have unlucky streaks!) not to achieve better results. Your future self might have better hooks, more exact imitations, better drag-free floats, and better luck, and in the same day would have caught 20+. But if all you wrote was "landed 2 fish", then your future self doesn't know the difference between these two days, and is likely to assume the fishing sucked.
Same goes with pattern. It might be worthwhile to mention that the comparadun worked better than the catskill tie. But fish change with pressure and what other guys are using. Likewise, do you really know what they were keying on? Maybe that catskill you tried had some hackle out of place, the wrong length of tail, or an imperceptible UV glow, or you gobbed it with too much floatant, something oddball could have turned those fish away. And maybe your future self ties better flies, with a different brand of thread and dubbing. If you notice a preference for "in the film" vs. "high floating", well, then thats more important to your future self.
