If you're feeling too sore to move the day after a workout and can't even begin to attempt another minute of exercise, it's time to rethink your strategy. Shaar says sometimes it's beneficial to end a workout a little prematurely. "It enables me to end on a high note while I still feel very energetic, it enhances my post-workout glow, and it truly makes me look forward to my next session," she says.

Spangler says this method is an especially great idea when you're just starting out. "People start giving 100 percent and then they burn out quickly," Spangler says. "You're better off starting slowly with something like strength training or a yoga class [if you're not used to exercising]. Then build up your stamina and try more challenging workouts."

While this excuse will get you out of some workouts, our experts say there are plenty of exercise methods that can actually help heal old injuries and ease body pain, which in turn can motivate you to work out even more.

If you have back, hip, or leg pain, Pilates can help strengthen your core and improve your range of motion, says Michael Leonardi, administrator at the Neurac Institute for Physical Therapy. "If you fix little nagging injuries, you can get better workouts and enhance your performance," he says.

Cabral recommends starting with stretching, interval training, and resistance training if you're overweight. Extra weight can often mess up your alignment and cause injuries if you start running or doing hard-core aerobics.

Frustration is totally normal when you're working toward a fitness goal. The key, however, is to understand how to work through it. First, Gervais says we need to realize that frustration is a secondary emotion; it's actually a reaction to hurt or fear. So ask yourself what you're afraid of, he says. "You may be fearful of some outcome, like 'All the work I'm doing is not going to pay off' or 'I'm not going to get where I want to be,'" says Gervais. When you know what's really bothering you, you can address that and make adjustments in your fitness plan to work through it.

Gervais also points out that frustration isn't actually bad. "Frustration is where growth takes place," he says. "Most people are trying on a regular basis to be comfortable, but that's not where true learning and growth happens. When you're frustrated, know that you're on the edge of the comfort/uncomfortable zone."

When you do get in that zone, Vokshoor says it's a good idea to stop, take a deep breath, and have a conversation with yourself. Remember that you're doing something great for you. Carrie Cheadle, MA, a sport and exercise mental skills coach, says it's also helpful to visualize yourself feeling strong and excited and motivated -- even when you're actually feeling the exact opposite. "Just by doing this you are planting those seeds in your brain and making it more likely that you will [continue]," she says.

C'mon, you know this isn't true. But you may feel like it is if you don't really understand what you're doing. "I teach clients why they're doing the exercises, why they need to eat a certain way, so they understand what they're doing," says Cabral. "When you don't know why you're doing something, you'll fall off the wagon at the first sign of a set back."

To help you understand why the heck you're doing all those lunges, ask your trainer (if you have one) what the benefits of each exercise are. Or if you're going at it alone, check out fitness websites and magazines for more information on the benefits of exercise, both in terms of overall health and weight loss. It's a lot easier to trade an Oreo for a run if you fully understand what each does for your body.

This one has some genuine validity. After all, "your brain cannot be starving and have an effective workout," says Vokshoor. He adds that "state of mind dictates everything about physical performance." That's why proper nutrition and getting enough sleep is so important. Vokshoor says to think of your brain as having a gas tank; just like your car, it can't run on empty.

So yes, sometimes you may have to skip a workout so you can catch a few extra z's or whip up a healthy meal. But don't use lack of sleep as a consistent excuse. Instead, make it a priority to get a full night's rest so you can perform at your best. And in a circle of life kind of way, keep in mind that exercising during the day can actually help you sleep at night. So even if you don't feel like jogging 10 miles, do a small amount of exercise, get a better night's sleep, and be ready to tackle more tomorrow.