There are many apps that you can use to track your mood on a daily basis. My personal favorite is Moody (iPhone). It is very simple and intuitive. It allows you to log your current mood by number, take a note, and even add a photo if you want. Open up a graph of your entries to identify trends across time (and export the graph to a file that you can send to your doctor). You can also configure the app to remind you to log your mood every day. You have to upgrade to the Premium version to log more than 15 entries, but the cost is just a one-time fee of $4.99.