Exercise is a KEY to happiness. Research shows that people who exercise are healthier, more energetic, think more clearly, sleep better, and have delayed onset of dementia. They get relief from anxiety and mild depression. They perform better at work.

Also, although it's tempting to flop down on the couch when you're feeling exhausted, exercise is actually a great way to boost energy levels. Feeling tired is a reason to exercise, not a reason to skip exercise.

But even when you admit that you'd feel better if you exercised, it can be very hard to adopt the habit. My idea of fun has always been to lie in bed, reading, preferably while also eating a snack – but I've managed to keep myself exercising over the years by using all these tricks on myself: 1. Always exercise on Monday. This sets the psychological pattern for the week. Along the same lines…

2. If at all possible, exercise first thing in the morning. As the day wears on, you'll find more excuses to skip exercising. Get it checked off your list, first thing. It's also a very nice way to start the day; even if nothing else goes right, you've accomplished that.

3. Never skip exercising for two days in a row. You can skip a day, but the next day, you must exercise, no matter how inconvenient. (Lately, I haven't been following this rule, and it has really affected my routine for the worse. I'm going to re-double my commitment to it.)

4. Give yourself credit for the smallest effort. My father always said that all he had to do was put on his running shoes and close the door behind him. Many times, by promising myself I could quit ten minutes after I'd started, I got myself to start – and then found that I didn't want to quit, after all.

6. Exercise several times a week. If your idea of exercise is to join games of pick-up basketball, you should be playing practically every day. Twice a month isn't enough.

7. No time to exercise and take a shower? Find a way to exercise that doesn't require you to shower afterward. Each week, I have a very challenging weight-training session, but the format I follow doesn't make me sweat. (Some of you are saying, "It can't be challenging if you don't sweat!" Oh yes, believe me, it is.)

8. Look for affordable ways to make exercising more pleasant or satisfying. Could you upgrade to a nicer or more convenient gym? Buy yourself a new iPod? Work with a trainer? Get a pedometer to keep track of your walking distances? Exercise is a high life priority, so this a worthwhile place to spend some money if that helps.

9. Think of exercise as part of your essential preparation. For times you want to be in especially fine form, whether in performance (to be sharp for an important presentation) or appearance (to look good for a wedding) or mood (to deal with a stressful situation), excercise is the answer. In college, my roommates and I always made sure to exercise the day of a big party. Studies show that exercise does help.

10. Remember one of my favorite Secrets of Adulthood (courtesy of Voltaire): Don't let the perfect be the enemy of the good. Don't decide it's only worth exercising if you can run five miles or if you can bike for an hour. I have a friend who scorns exercise unless she's training for a marathon — so she never exercises. Even going for a ten-minute walk is worthwhile. Do what you can.

11. Suit up. Even if you're not sure you're going to exercise, go ahead and put on your exercise clothes. Pack your bag. Put the dog's leash by the door. Get prepared. If you're ready to go, you might find it easier just to go ahead and exercise. Sometimes, a very trivial thing — like not knowing where your shoes are — gets in the way.

12. Don't kid yourself. Paying for a gym membership doesn't mean you go to the gym. Having been in shape in high school or college doesn't mean you're in shape now. Saying that you don't have time to exercise doesn't make it true.

People often ask me, "So if I want to be happier, what should I be doing?" and I always say, "The first thing to do is to make sure you're getting plenty of sleep and some exercise."

I know, that answer doesn't sound properly transcendent and high-minded on the subject of happiness, but research shows that you'd be wise to start there. And I've found that if I'm feeling energetic and well-rested, I find it much easier to follow all my other happiness-inducing resolutions. ____________________________ Gretchen Rubin is the author of the #1 New York Times bestseller, The Happiness Project — an account of the year she spent test-driving the wisdom of the ages, the current scientific studies, and the lessons from popular culture about how to be happier. On her popular blog, The Happiness Project, she reports on her daily adventures in the pursuit of happiness.

She is the author of several books, including the bestselling biography, Forty Ways to Look at Winston Churchill. Rubin started her career in law, and she was clerking for Justice Sandra Day O'Connor when she realized she really wanted to be a writer. She lives in New York City with her husband and two young daughters. Follow her on Twitter, @gretchenrubin, or join the Facebook Page.