Just as your body needs to detox if you overindulge in alcohol or unhealthy food, your brain needs to detox from negativity, overwork, and anxiety. That's the word from Jason Selk, bestselling author of Executive Toughness and director of mental training for the St. Louis Cardinals from 2005 to 2012. (During that period the team won the World Series--twice.)

"The main reason our brains need to detox is that thoughts that come normally can be toxic," he explains. Humans appear to have a biological tendency to focus on problems rather than on the good things in our lives. For instance, he says, "oxygen is one of the most valuable resources for human beings, but we don't think about how we have all this abundant oxygen. We relate to each other the same way. Read the newspapers, watch the news, or listen to the conversation around the office. Everyone's talking about problems instead of solutions."

You can reprogram your brain toward the positive and become both happier and more productive in the process, Selk insists, with a 21-day mental detox program. For the next three weeks, try out these steps:

1. Pick your top three to-dos and do them first.

Take some time at the start of your day to plan out what you'll do, and select the three most important (not most time-sensitive) items on the list, Selk advises. Then do those tasks first.

"There's a great Stephen Covey quote: 'The noise of urgency creates the illusion of importance,'" Selk says. "We have a human tendency that the urgent stuff gets our attention. But doing what's urgent as opposed to what's important leaves you feeling like you performed less than your potential. And people who get all the important things done, but not all the urgent things done are more mentally healthy than their counterparts. So ask yourself, 'What are the three most important things I need to get done today?' You need to have that process daily."

Starting with these tasks can help you with the rest, he adds. "The important tasks are usually the hard ones, so getting those done early is like getting points on the scoreboard. It can inspire confidence and give you momentum."

2. Look for small improvements and log your successes.

"People have a perfectionist mentality," Selk says. "If you judge yourself against perfection, discouragement is inevitable. And once a person gets discouraged, it's really difficult to be motivated." So fight that human tendency by taking note of small successes, such as eliminating three of the 25 items on your to-do list, he advises.

"It's an incredibly foreign process for most people to recognize on a daily basis what they've done well," he adds. "We typically focus on all the stuff we've screwed up, and expectancy theory states that whatever we focus on will get bigger. So it's counter-productive for us to keep focusing on what we're doing incorrectly." Instead, he advises, take one minute a day to recognize three things you've done well. "You don't have to cure cancer for it to qualify. As the great coach John Wooden says, little things done well create excellence."

3. Commit to finishing what you start.

"Some people get a job 90% done. Then they feel somewhat satisfied so they kind of leave it there," Selk says. "You're so much better off if you can take that task to full completion. Of course, with some tasks that's not possible in a day, so break it into what's manageable. If you're trying to paint your house, you can't paint your whole house, but commit to getting one room fully painted today. Then, instead of doing three and a half walls, you get that room finished."

This approach leads to a better mental state, he says. "The things that sit on our to-do lists feel like unfinished business and they create stress and anxiety."

4. Commit to getting exercise and rest.

Both are very important for the health of your brain, Selk explains. Studies such as these demonstrate that regular exercise prevent losses in mental function that both aging and stress can create.  So Selk advises fitting at least three half-hour workouts a week into your schedule. "The productivity and energy that exercise brings far outweighs the loss of a half hour," he says.

Rest is just as important to mental function, he adds. "The brain really is like a battery--it only requires one full night's sleep in order to recharge." So working hard and depleting that battery is fine as long as you get at least one good night's sleep every few days, he says. The same goes for mental rest while you're awake. "Take out your calendar and schedule one day of rest for every seven-day cycle for the next three weeks," Selk advises. "You will find yourself more productive and less mentally exhausted for the other six days."