Aside from our mothers’ nagging, our guilty consciences and Jiminy Cricket, willpower is the best thing we have to help us do the right thing when it comes to our money.Accomplishing financial goals, from deciding whether to buy the perfect pair of wedges or start funding a fabulous honeymoon, often requires choosing between now and later, and needs and wants. Making the correct choice, however, demands the willpower necessary to turn down passing pleasures in favor of the long-term goals you want to achieve.Luckily, it turns out that willpower functions exactly like a muscle—which means that while it can become tired and sore, it can also be strengthened, helping you reach your goals more quickly and efficiently.Read on to find out how willpower works in your brain (no Ph.D. required!) and how you can build it up to achieve your goals—financial or otherwise.MORE FROM LEARNVEST: Your Money Questions, AnsweredWhat Willpower IsEarlier this year, Roy F. Baumeister, a research psychologist, and John Tierney, a New York Times science writer, published a book called “Willpower: Rediscovering the Greatest Human Strength,” detailing Baumeister and other scientists’ research regarding willpower, self-control and motivation.Studies show that willpower—the determination to accomplish a difficult task, stay on a plan, break a bad habit and more—exists, but people have only one finite reserve of it. As a result, if you’re trying to accomplish a lot of goals all at the same time (New Year’s resolutions, anyone?), you’re more likely to fall short because your willpower is getting exhausted by everything that’s challenging it (that chocolate bar, the impulse to bite your nails, the urge to have a cigarette or a second drink).That’s why, the authors point out, marriages often go bad when stress at work is high—people use up all of their willpower at the office, and as a result have none left to avoid nagging their spouses about not doing the dishes or paying the bills on time.Even more groundbreaking?It appears that willpower is actually dependent on glucose—the sugar that our bodies convert from food. Basically, if willpower were a car heading to a destination, glucose is the gasoline fueling the journey … and each mile empties the tank of glucose a little more.What’s interesting is that it’s not just the resolve it takes to make “good for us” choices that wears out our reserves of willpower, it’s the mere act of deciding. The more decisions we make (about whether or not to go to the gym or how we should respond to our boss’s email), the quicker our stores of glucose are depleted, and the less willpower we have to make good decisions.How This Affects Your FinancesWillpower has serious effects on your finances, from controlling your little splurges to helping you decide whether to buy the car with only the basics or with the sunroof, leather interiors and cool trim package.Want to know how depleted willpower can negatively affect your financial wellbeing? Here are a couple of examples provided by Baumeister and Tierney:1. Avoiding Making DecisionsWhen our willpower is tapped, researchers find, people take the path of least resistance: that is, they avoid making more decisions.In one study, people were told that they had $10,000 that they did not need in a low-yield savings account. They were then given the opportunity to make a good investment with average risk and an above-average rate of return.When subjects had their willpower depleted in a test setting, they decided to leave their money where it was, to avoid having to make the decision to invest. This made no logical sense in terms of their finances, because the savings account was not going to make as much money as the investing option.In fact, given the good investing choice, the people who had full stores of willpower decided to invest.MORE FROM LEARNVEST: Why Women Should Be More Open About Salaries2. Ignoring the Relationship Between Price and QualityWhen you’re shopping, you’re making constant decisions: This shirt or that shirt? New espadrilles or wedges? And so on.But one of the most common decisions facing shoppers involves the relationship between quality and price. One example the authors use to illustrate the relationship compares two bottles of wine: a $20 bottle and a $100 bottle. While a $100 bottle is usually better than a $20 bottle, they ask: “Is it five times better?” (The answer is probably no.)Because of this dynamic, shoppers need to figure out exactly where the gains in price start rapidly outstripping the improvement in quality. However, studies show that when willpower is low, a shopper is less able to understand this dynamic, and will only look at one factor, either deciding to go with the cheapest option—no matter how poor quality the item is—or with the most expensive—no matter how absurd it is.3. Being Easily Manipulated by Salespeople Baumeister and Tierney point to a study done in German car dealerships showing that people who were asked to make many decisions—and therefore had their willpower depleted—were more likely to accept the option recommended by the salesperson. (Which, if you’ve ever dealt with a car salesperson, you’ll know is generally the most expensive option.)If subjects were asked to make dozens of small decisions about buying a car, from the color to the interiors to the navigation system, they had very few defenses left to resist the salesman’s later recommendation of the most expensive engine available. Sound familiar?4. Making Short-Sighted Decisions About MoneyWhen given the option between $100 now or $150 a month later (and you don’t have to do anything extra to earn the difference in payout), the obvious “better” choice is to wait for $150.But in one study, when subjects had their willpower depleted, they generally chose the instant cash; in other words, they didn’t have the willpower to wait a month.How to Strengthen Your Willpower and Avoid Bad Money Decisions:Feed Your Brain In the last example ($100 now vs. $150 later), the results were turned around by one small factor: glucose. If subjects who had their willpower depleted were fed (supplying their brains with a new source of glucose). their willpower replenished itself, and they were able to make the better financial decision, choosing the $150 option.So how do you boost your glucose levels? Eating low-glycemic index foods like vegetables, nuts, cheese, meat, raw fruits and olive oil feeds your brain a steady stream of glucose, while quick sugars found in candy and white bread cause a spike and then a crash. Choosing low-glycemic foods, therefore, will give you more willpower over a longer period of time.Additionally, try to make important financial decisions—be it choosing an insurance provider, rebalancing your portfolio or house hunting—right after a good meal. This will ensure that your brain’s source of willpower is replenished, and you’ll be able to make the best decision … rather than the best decision for the salesperson at hand.MORE FROM LEARNVEST: Why I’m CheapMonitor YourselfSelf-awareness helps bolster willpower and keep you on the right path to reach your goals. By seeing exactly where your money is going, you’re less likely to make decisions with negative financial consequences. Research showed that using budgeting software (like LearnVest’s My Money Center) helped 80% of subjects control their spending, especially in categories like groceries, restaurants and credit card finance charges.Making these cuts in your lifestyle choices can help you put more money toward your financial priorities to help you achieve your goals. The obvious answer? Start tracking your spending today using the My Money Center, and start strengthening your willpower to avoid the types of spending that can derail your budget.Automate Your Finances If decision-making depletes your willpower to make good decisions, then take decision-making out of the mix altogether. By automating your savings and retirement contributions, you won’t be confronted with the decision to put that $200 toward your 401(k) or a night out on the town. Here’s more info on how to automate your savings.Plus, that leaves you with more willpower to make decisions in other areas of your life; we bet you’ll never snap at your significant other again.