Mirrors give you feedback. When you look at your facial expression in a mirror, you have more data to answer the question, "Do I look like I am joyful or enthusiastic right now?"
If the answer is, "Yes," terrific. If not, don't answer, "No." Rather, reply to yourself, "Not yet." This gives your brain a message, that eventually you will be more enthusiastic than you are now.
Enthusiastically wave to yourself in a mirror. Even if you didn't feel enthusiastic before you started, the feedback you get will bring a real smile to your face if you allow it to.
"But I don't have anything to be enthusiastic about," some people argue.
You can always be enthusiastic that you are alive, that you are breathing, that you can see. As you wave to yourself in a mirror, be enthusiastic that your brain is creating the chemicals that naturally make you feel good.
Regularly practicing waving enthusiastically is a simple tool that will be powerfully effective. If you are serious about becoming more enthusiastic, you will make this a habit until you will no longer need to make an effort to see the enthusiastic look when you see yourself in a mirror.
(From "Enthusiasm: formulas, stories, and insights" (2002) pp.128-9; available from http://www.artscroll.com)