Want to feel happier, emotionally healthier, and more optimistic? When you first wake up in the morning, think of three things you're grateful for.

You've heard a lot about gratitude this week. Gratitude makes you happier, more resilient, more likeable, and can literally add years to your life. You know gratitude is good for you. But how do you add more gratitude to your life? If you're not already feeling grateful, how do you start?

If you find this difficult, you're a lot like me. I tend to focus on the things I worry about rather than those that make me happy. That doesn't make us bad or pessimistic people, it just makes us human. Because noticing threats quickly was key to our ancestors' survival, the human brain is literally better at paying attention to what's bad rather than what's good. Don't blame yourself if you have a negative outlook--blame evolution.

But that negative outlook doesn't always serve us well in the modern world, so it's worth doing something to change it. Change begins with a simple practice, one of the very few I've found I can stick with over time because it only takes a few seconds and you do it every single day.

First thing in the morning, when you first open your eyes, or realize that you're awake, list three things to yourself that you feel grateful for.

That's it. That's all you have to do to change your attitude for the better. Because you're priming your brain with positive information at the very start of your day, and placing your attention on the good in your life rather than the bad, you are gently reprogramming your brain to have a more upbeat world view.

Do this before your feet hit the floor and especially before you turn on your smartphone or other mobile device. Once you've started reading your messages, checking Facebook, skimming the news, or looking at the weather report, you're into your day and your gratitude exercise will be less effective. It's also much more likely to get forgotten.

There are no rules about what you can be grateful for. In fact, there's only one rule to this practice: Be honest with yourself. You never have to tell anyone else what you're thinking. One recent morning I woke up and found I was grateful that my husband and I had had a huge and fun party the previous day, and equally grateful that the party was over and done with. Most mornings I wake up grateful for my husband and our cats (both of whom are invariably in bed with us). Some mornings I wake up grateful for a little time to myself.

So go ahead. What do you have to be grateful for? Make a mental list of three items every morning before you get up. That's all you have to do to reprogram your mind toward gratitude--and give yourself a happier day.