By Anne Fisher
January 2, 2013

FORTUNE — Eat too much rich food or overindulge in liquid cheer, and your body will tell you it needs a break — lots of water, lighter fare, and maybe a few good workouts. According to Jason Selk, before you tackle new challenges in the year ahead, “the mind needs detoxing too. We cram it full of disturbing media images. We don’t give it enough rest. Or we strain our brains to the max with work worries and deadlines.”

Selk was director of mental training for the St. Louis Cardinals in 2006, when the team won its first World Series in 20 years, as well as in 2011, when the Cardinals won it again. He also coaches executives at companies like Wells Fargo (WFC), Enterprise Rent-a-Car, Northwestern Mutual, and financial advisors Edward Jones on how to get the most out of their brains.

“Mental detoxing is necessary for optimal performance,” says Selk, “whether you’re a pro athlete or an office worker.” So how does he recommend you do it? For the next 21 days, try these six steps:

1. Do 30 minutes of exercise three times a week. Selk points to reams of medical research confirming that exercise helps head off the slowing down of mental function that can result from stress (and from aging). “Schedule at least three cardio or weight-lifting workouts every week for three weeks,” he advises. “The productivity and energy that exercise brings will far outweigh the loss of a half hour from your workday.”

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2. Plot each day in advance. “Organize each day before it starts, even if you have to get up earlier in order to do it,” he says. “Choose three daily tasks that will have the greatest influence on your performance and success.”

3. Do your top to-dos first. “Most people do this backwards. They focus first on the unimportant tasks and save the most crucial activities for last,” Selk observes. The problem with that, he says, is that “you’ll need the greatest energy and focus at the end of the day, when you’re already tired from spending hours on low-priority tasks. By getting the most important items done first, you create mental energy and momentum for the rest of the day.”

4. Finish what you start. “Unfinished projects leave us feeling self-critical and hassled. As you tackle a project, commit to concentrating on it until it’s done,” Selk suggests. “If it’s a large project, break it down into manageable parts that can each be finished in one sitting.” Avoid multitasking if you can, and shut out interruptions and distractions as much as possible.

That’s often easier said than done, of course, but it’s worth a try: “Developing the self-discipline of a finisher will help you feel more satisfied and mentally relaxed at the end of your workday.”

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5. Aim for small, continuous improvement. “One sign that you need to mentally detox is feeling hopeless, overwhelmed, or burnt out,” says Selk. “This comes from focusing on the end result, and how far away it seems, rather than looking for small signs of progress along the way.” So, for 21 days, “remind yourself to seek out any improvement in the situation, small or large. When you get into the habit of seeing improvement, you become more optimistic and less discouraged.”

6. Recharge your mental battery. “The overloaded brain needs rest in order to function optimally,” Selk notes. “The brain really is like a battery, and it only needs one full night’s sleep in order to recharge.” It’s okay to work hard and be mentally tired, even several days in a row, as long as you get one solid night of eight hours of sleep every few days.

“Mental rest during the day is also important,” Selk adds. “Take out your calendar and schedule one day of rest for each seven-day cycle for the next three weeks. You will find yourself more productive and less mentally exhausted during the other six days.”

The reason for doing all this for 21 days is that, according to recent neurological research on brain plasticity, that’s how long it takes to replace old habits with new ones. Just for good measure, though, you might think about keeping up these behaviors — or any other New Year’s resolution you’ve made — for a full 30 days instead.

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When executive coach and author Rory Vaden commissioned research firm Kelton to do a survey on New Year’s resolutions, the results were intriguing. One in two Americans made a resolution in 2012 and, predictably, nearly half (48%) of those quit trying, at some point, to keep it. Many gave up before January ended. But among those who persevered for 30 days or longer, more than three quarters (76%) were still sticking to their resolutions.

“It’s tough to cultivate the self-discipline to make real change,” says Vaden, author of Take the Stairs: 7 Steps to Achieving True Success. “But if we can just make it through the first month, we’re three times more likely to make a permanent change.”

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