mate in my diary i put in the following items:
* Date (Obviously)
* Time i fished
* Bait used
* Berley used
* Tides for the day (high and low)
* Sunrise / Sunset times
* What moon phase we are in
* Water temperature
* what the barometic pressure was for the day and how it changed throughout the day
*what the general weather was for the day and how the weather was for the past few days (i.e. was there any rain in the previous few days etc.)
Then i detail each fishing locations i fished, including any specific structure i was fishing, water depth etc, along with what time i fished these spot I detail what i caught at these spots (including undersized fish.
I try to fish the same river system so that i can keep that variable constant and so that i can learn what variables affect what location.
Although it is hard to get your head aroud all the stats you collect, but in the 3 years of taking data of my fishing i have found that fishing in early morning with low light is the better fishing time. Fishing during the middle of the day is generally poor unless there is heavy cloud cover or a breeze that breaks the water surface. Also i fish the hacking, which is generally very clear, i find that a bit of rain murking up the water fishes better than clear water.
Im slowy getting to see some trends though it takes a lot of effort to learn what variables are most imporant to your area. Keep collecting the data mate and you should start seeing results.