Really just figure out what you want to do with the data. Tracking just round count is easy. Some of the data I know some folks capture is.

1. Rounds shot
2. Temp
3. Wind speed and direction
4. Hit position with some sort of grid reference
5. date and time
6. What you were working on skill wise
7. Other thoughts and notes
8. Altitude

Most of the log books tend to be restricted to a specific weapon. Eagle makes one that I know of, it's a right-in-the-rain dealio hence the cost.

WWW.Snipershide.com has some good info about this as well.