Self-control. It helps you resist the urge to yell at your boss. It keeps you from eating a whole pint of ice cream. And you summon it up when you want a pair of shoes, TV or car that you can't afford.

Discipline is critical to living within your means. Sometimes, though, we all fall short. Here are a few basics about self-control and what you can do to make it work for your financial goals. A common misconception is that strong self-control is something people either have or don't have. The good news is it's not the fixed ability that most people think, said Heidi Grant Halvorson, a social psychologist and author of "Succeed: How We Can Reach Our Goals." "It's better to think about it as a muscle that some people have developed and some people have not," Halvorson said.

Self-control, like a muscle, fails when it is tired. You have only a limited amount of self-control. Anxiety, depression, fatigue, stress, anger and frustration tap out those reserves. Making many decisions also erodes self-discipline, Halvorson said.

So after a long day at the office making decisions, you are more likely to do impulsive things like overeat, drink too much, lose your temper or spend money. The same is true if you spent the day at the mall evaluating what to buy or not buy. At the end of the trip, your self-control may be weaker.

The key to not spending money impulsively is to recognize when your willpower is low. The critical next step is to have a plan for times of weakness. Before you walk into a store, know what you are going to buy and how much you are going to spend. The more specific you are, the more likely you will stick to the plan. Shopping lists or spending limits help check your spending. Take a cooling-off period, wait a day to make a big purchase or just take a deep breath to reconsider.

Practice small acts of self-control: exercise daily, give up alcohol for a week, or get up at the same time each day for a month if you are prone to hitting the snooze button.

These minor acts can strengthen the part of your brain that handles self-control and give you the resolve to do more, Halvorson said.