If it's so hard to keep to the 50% limit, why do it? Several good reasons:

  • It gives you flexibility. Your income could drop by half and you'd still be able to pay your essential bills. When your must-haves eat up more of your income, you have less ability to cope with setbacks such as layoffs, reduced work hours or unexpected expenses.
  • It helps you figure out what you can and can't afford. If you're considering adding a loan payment or other contractual obligation to your overhead, you simply check to see if it would push you over the 50% mark. If not, you can consider adding the payment; if so, you don't.
  • It gives you balance. Limiting your overhead allows you to have money for the pleasures in life, such as dinners out and vacations, without stress. It also allows you to get out of debt and save for your future.

So what should you do if your numbers are out of whack? Remember that the 50/30/20 plan is a goal to work toward, not something you'll necessarily achieve overnight. And if you're already in financial crisis -- you're unemployed, for example, or suffering through a disability -- true balance may have to wait until the crisis has passed.

But here are some places most people can tweak:

  • Food. You've got to eat, but most of us could trim our grocery bills, often substantially, without too much effort. Plan your meals, cook from scratch, use up leftovers, clip coupons -- you know the drill.
  • Utilities. You want the lights to stay on, but the air conditioner doesn't have to blast 24/7.
  • Transportation. More carpooling and public transportation, less time alone in your car. Your car payment could be what's killing you.
  • Insurance. Higher deductibles can help reduce your premiums, as can shopping around and taking advantage of all available discounts. Ditch insurance you don't need, such as life insurance if you don't have financial dependents, or collision and comprehensive coverage on a clunker.
  • Ditch the contracts. Early termination fees might make canceling your cell service too expensive, for example, but once your contract is up, consider switching to prepaid or pay-as-you-go service. Unless you're a real gym rat, gym contracts are another expense to shed as soon as you can. Consider paying by the visit or signing up at the local Y, which offers monthly billing without long-term contracts.

Other costs are tough to winnow but may be worth the effort. If you're paying too much for housing, you may need to consider a roommate or a move to cheaper quarters. If your child care expenses are eating you alive, brainstorm other alternatives.

You may think it's your income, rather than your expenses, that's the problem. That could be true, and if you can boost your income, go for it. But people can balance their budgets and save money on virtually any income.

If it's your debt that's unmanageable, you may need to consider some more drastic solutions -- credit counseling, debt settlement, bankruptcy or foreclosure. Some bills are simply impossible to pay, despite your best efforts, and you may need help or a fresh start.

Once you get back on your feet, though, the 50/30/20 plan can help you stay there.

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.