Reposted to remind myself.
From an old journal entry:
I was lying awake thinking this morning and remembering that one of the strange-but-true ways to be happy is simply to pretend you are. Let me tell you - no matter how intelligent and rational you are, your subconcious is still VERY gullible. I first observed this phenomenon when I was a camp counselor. At that age I was always disgustingly cheerful, and I was a morning person to boot. This was the age at which I learned to steer clear of people who had not yet had their morning coffee and that my bouncy cheerfulness can be physically painful to those people even though I haven't touched them at all ("The light! It burns us!"). Anyway, one day I was having a really really bad day. I felt like crawling into my cot and crying, but I had to return to whatever activity it was and I needed to be cheerful for the kids (because it's a whole lot easier to lead kids when you're happy because they imitate you without realizing it - gee - looks like I've forgotten that, good thing I told you this so I could remember). So on my way down the hill from my unit I started pretending to be my normal, bouncy, cheerful, in-your-face-happy self. I bounced. Not because I was happy, but because I was pretending to be. I put a happy, convincing bounce into my step, because children are not easy to fool. I put my foolish-with-happiness smile on and made it go all the way to my eyes. Would you believe that within seconds I felt better - and not only better but actually, truly, bouncy-joyful? No, of course you wouldn't believe me. I highly recommend you try it for yourself sometime; it's just bizarre.
Mellryn used this phenomenon in a scene in her book (which is being shopped to publishers now - I'll let you know when you can buy a copy; this is a GREAT book and just by reading it you can tell that it's the first in a great series). I am going to oversimplify this scene greatly for you: When the female protagonist nearly kills someone by accident and is starting to crumble under the horror of having done so, the protagonist's mentor makes her do jumping jacks. Why? Because acting bouncy absolutely forces you to feel bouncy.
Remember - pretend to be happy, both facially and physically, and you will become so.