One of the best things to do is to write it all by hand. It is so easy to make little notes and sketches of places you have caught fish.

The best logbook to get is a Land Surveyor's note book. It's a small notebook that is water resistant and lasts a long time!!! The brand I have is "Write In the Rain" or something like that. It's small enough to fit in my tackle bag and it's a great way to look back at years past and where I've caught my fish.