Eat Healthy - It would surprise most people how much your diet can affect your outlook. But the old saying "Garbage in, garbage out" really applies. Tanking up on trash constantly makes your body feel tired, sluggish, and can really bring on a case of the blahs. The healthier you eat, the better you will feel. And it doesn't have to cost an arm and a leg either. I changed my diet and found out I was spending less on healthy foods (that taste just as good, I might add) than I was on junk.
Exercise - It plays off the point listed above, but it still holds true. You don't have to be an athlete or run a marathon, but you need to get up off your butt and move around some. Spend some time in the sunshine, take a walk, anything to make your heart beat, your lungs expand, and to put your body in motion.
Be Passionate - About SOMETHING. Anything. Find your passion, and follow it. I'm not saying to quit your job in your quest to become the world's finest cheese-maker (though if you think you can, go for it!), but find something that stirs your soul. For some its painting. For me it's writing and music. I can't live without either one. Well, I could, but I'd be one miserable bastard if I did.
Ignore Stereotypes and Fads - ...which is a less Pollyanna way of saying be yourself. Living your life living up to what Vogue, Cosmo, and People declare are the "in" fashions and trends is a quick way to make yourself feel inadequate. Don't believe me? Look at the covers of those magazines and realize that those women are being force-fed as the ideal of "beautiful." Sorry, but unhealthily-skinny women just don't do it for me. As for fashion, I wear what I like (which is usually a Hawaiian shirt with sneakers and jeans). Wear what you like. Be you, not who someone else wants you to be.
Never let anyone determine your worth - No one needs any other solitary person to be whole. Too many people are part of bad relationships or are made to feel less than their worth. Make yourself happy and realize that you are a whole person. Once you realize that you don't need another person to make you whole, you'll realize that you enjoy other people more, and that they enjoy you.
Love Yourself First - Boy, that's a hard one. For someone who came from a veritable plethora of neuroses and self-loathing (me) to somehow come to terms with, and genuinely like, himself is a huge step. But the old saying of "You can't love someone else until you love yourself" is true.
Laugh - Best medicine and all, but a good hard laugh has been proven to have all kinds of psychological and physical benefits. Just never forget to laugh at yourself and with others. Never take yourself too seriously.
I hope someone who reads this finds it useful. Please, feel free to leave your own hints for happiness as comments. Notice, I never said turn to Christ/Buda/Allah/The Flying Spaghetti Monster. Happiness comes from within.