When it comes to advanced calculus or the Sunday crossword puzzle, there's no doubt that your mind is in charge. But when you're crunching that 100th sit-up or scaling 10 flights of stairs—surprise—your brain is still boss. How fast, how long, and how powerfully you perform is, in fact, mostly in your head.

For years, exercise scientists assumed that we—athletes, gym goers, fitness walkers—become weary and slow down when our muscles run out of oxygen. Not so, says researcher Timothy D. Noakes, M.D., a professor of exercise and sports science at the University of Cape Town, South Africa. He points to his 2001 study that found when cyclists quit out of exhaustion, on average after 62 miles, their muscles still had plenty of oxygen left to continue pedaling.

Essentially, Noakes says, their brains turned on the discomfort before they actually ran out of fuel in order to help shield their bodies from potential injury.

That's not to say you can breeze through a workout simply by keeping your head in the game. Fitness is still key, but having the proper mental tools can give you that extra something to push you to complete your goals—whatever they may be, says Steven Ungerleider, Ph.D., a sports psychologist and author of Mental Training for Peak Performance.

Prevention talked to fitness coaches, sports psychologists and exercise physiologists to find out how to clear the mental hurdles that most often keep people from reaching fitness targets. Use these tricks and soon your brain and body will be working in sync to get you in the best shape of your life.

"I'm tired. I can't go on!"

The Get-Over-It Fix: Pretend there are springs on your feet.

If you dream it, you can do it. Seriously. Research suggests that our bodies can't distinguish between something we've drummed up in our minds and something we've really done.

When subjects are wired with electrodes and asked to imagine that they're running a race, muscles contract in much the same way they would if the people were actually moving, finds researcher JoAnn Dahlkoetter, Ph.D., a sports psychologist at Stanford University. (If you've ever felt winded after waking from a chase sequence in your dreams, this makes sense to you.) The way to make this phenomenon work to your advantage? Positive imagery.

"I remember training a woman at the track who was so tired she could hardly move," Dahlkoetter recalls. "When I asked her what sort of pictures popped into her head when she was working out, she admitted to thinking of herself as a fat slug." Dahlkoetter asked the woman to instead imagine that she had springs on her feet or helium balloons lifting her forward.

"Changing her thought process changed her whole workout experience; suddenly she felt light on her feet and able to move faster," Dahlkoetter notes. "She was energized by her workout."

If you're not into spring or balloon fantasies, pretend that the walker or runner a few paces ahead of you has a powerful magnet on her back that's pulling you along.

"Once you catch up, take the magnet and put it on the back of the next person in front of you," Dahlkoetter says. "When no one is ahead of you anymore, envision the magnet at the finish line, effortlessly drawing you toward victory."

"I have no time."

The Get-Over-It Fix: Plan on paper.

You're busy. We know. But you may have more time for fitness than you think: Americans have twice as much leisure time as we believe we do, found a 2004 Harris Interactive survey of more than 1,550 people.

"We average 35 to 40 hours a week of free time," says Geoffrey Godbey, Ph.D., a professor of leisure studies at Pennsylvania State University. "The catch is that the time comes in small chunks."

To truly take advantage of those bursts of time, set—and write down—superspecific workout goals. "About 90% of the research out there has shown again and again that goal setting has a very positive effect by increasing motivation and persistence," says Aimee C. Kimball, Ph.D., director of mental training at the University of Pittsburgh Medical Center for Sports Medicine.

To help get you on your way, Kimball suggests keeping an exercise journal in which you record your long-term objectives ("I want to complete a marathon") and your daily targets ("I want to walk five miles today").

"Every night, write your detailed game plan for the following day and make sure you've taken into account any obstacles that may come up," Kimball says. Thinking ahead about potential roadblocks is the best way to sidestep them: That way, you'll know when to squeeze in your walk if that late night at the office sneaks up on you.

Finally, at the end of each week, note the problems you encountered, how you dealt with them and what you've accomplished. That should inspire you to carve out more time for fitness.

"I'm just not improving."

The Get-Over-It Fix: Cue the video.

Watching home movies from last Christmas won't help you reach your fitness goals, but tuning in to another kind of video may do just that.

"Seeing an image of proper form—a swim stroke, a tennis serve—has been shown to help the brain improve on what the body can do," Dahlkoetter says. As you watch, you imagine you're performing the action you see. All the while, electrical impulses travel from your brain to your muscles, helping your body remember how to perform properly.

Researchers from the University of Liverpool in England discovered that viewing a video of your own athletic feat improves performance by 29%, compared with an 8% change in a control group. According to Dahlkoetter, the same will happen if you watch highly skilled athletes compete. Chariots of Fire, anyone?

"Workouts are a chore."

The Get-Over-It Fix: Enlist a buddy.

As soon as you call an activity an "obligation," the fun gets sucked out of it. "To turn your workout into something you look forward to, make it your social time, too," says Jan Griscom, a certified personal trainer in New York City and former advisory board member of the American Council on Exercise. The easiest way to do that is to sweat with a pal.

University of Southern California researchers found that working out with a friend is the best predictor of exercise satisfaction. Another study revealed that when you train with someone you care about, you're more likely to stick with your fitness plan so as not to disappoint your partner.

Researchers at Indiana University followed 46 couples; some signed up for the gym together, some joined solo. After a year, the pairs who did it as a team had a mere 6% dropout rate, versus 43% for those who opted to go it alone.

"I'm too distracted to focus on working out."

The Get-Over-It Fix: Invest in an iPod.

"Listening to music shuts down the analytical side of the brain," Kimball says. "When you're engrossed in music, your mind can't tell you that you're tired or in pain or should be doing something else."

A recent study of 41 overweight women who participated in a 24-week diet and walking program found that those who listened to tunes of their choice lost twice as much weight as a group moving without a sound track.

"The music functions as a positive distraction, making you feel like you're not exercising as hard, so the women were able to do the workout more easily," says study author Christopher A. Capuano, Ph.D., an associate professor of psychology at Fairleigh Dickinson University.

"My enthusiasm is MIA!"

The Get-Over-It Fix: Love your reflection.

The mirror is a tricky thing. Your reflection can be either friend or foe, depending on your mood. But if you normally find your image pleasing, especially while flexing your muscles, working out in full view of yourself may give you a mental boost, says Jeffrey A. Katula, Ph.D., a researcher at Wake Forest University, who recently published a small study regarding the benefits of exercising in front of a mirror. He reported that if you feel good—pleased that you got yourself to the gym and that you're trying to improve yourself—then "the mirror can reinforce those positive feelings and potentially spur you to do more."

Give your mirror even more positive power by sticking a Post-It note on it that reads something like "I'm getting stronger every day," Ungerleider suggests.

"A constant encouraging reminder can help motivate you."