When I first started privately tutoring ESL students, I didn't have a system in place for keeping track of my lessons and topics covered. This wasn't a big problem for the first few lessons and when I had very few students. However, as the number of students grew and time passed, I wasn't able to remember everything. Especially since some students study English with me for a long time. It hasn't been unusual for some learners to work with me for over a year.
At some point I realized that I needed to keep better track of what had been covered. I thought of a checklist of sorts, but that didn't work very well, as different students had very different needs.
I have always prepared a one-sheet lesson plan for every tutoring session. Even if it is just a conversation class. During class, I use this piece of paper to make notes to myself about what has been covered and what needs to be covered in the future. Having a lesson plan also shows the student that you are a professional and they seem to appreciate the time you take to prepare for class. I also have a folder for each student.
At a minimum, I've found that it's at least important to know how many lessons I've had with the student. So at the top of every lesson plan, I have the student's name, the lesson number and the date. For example:
July 19, 2008
You usually never know for sure how long you'll be working with a student, so keeping track of your lessons can be very useful. I think it's essential (for my own sanity, if nothing else!).