My wife and I use a journal to keep track of our fishing trips, we keep track of conditions, time of year, hot bugs, where, etc, basically anything that could be used for future referrence.
Lately I have been thinking about finding an old laptop computer to use for this purpose, if I am driving home then my wife could input everything on the way, or vice versa.
Log books have been around for a long time but it doesn't seem like many people use them. I know many guides used them a lot. All of the fishing information is great but as headhunter stated, it is nice to read about the trip later. A log book to me has things like falling in the water, had a great lunch on the stream, my friend caught the biggest fish of his life, and so on. These are the things you will enjoy reading 10, 20 or 30 years down the road. Don't make it just a log book but make it a daily journal that you will live to cherish later in life.
Fyshstykr, I kind of favor using a computer also, since I can type faster than I write, and - guess what - I can read what I typed (unlike what I scribbled!). And you'd probably want to print out what you wrote from time to time anyway, which some printer programs allow to be printed in booklet format, making for a more useful size. It's a lot easier reading the written (typed in my case) word than off of a monitor anyway.
I just started this recently, but instead of a nice journal, I downloaded google earth and used pin points to mark my spots on the map. In the discription I put the time of year, weather, what flies worked best and what type of fish and how big.