I use Moodscope, which is a free online site, and uses a standard psychological scale to measure overall mood. It produces a nice graph that you can annotate. You can also have your mood reports emailed to "buddies" for support. I use this to keep my hubby informed of what my mood is like without having to tell him, "gosh, I only scored 35 today, I must be doing pretty crappy," directly. It helps.

I also use a paper-based form with a simple 9-point scale plus space to track meds, hours of sleep, etc. At the end of each month, I dump all the data into a spreadsheet. Yes, I'm a professional geek.

I also have a smartphone app called My Mood Tracker which also has nice graphs and a variety of useful options. But I haven't gotten in the habit of using it.