We all know we should save for retirement, but study after study shows that most Americans aren't saving enough.

As I wrote last week, a recent Hewitt Associates (NYSE:HEW) study found that workers with 401(k) plans will derive more than half of their retirement income from those plans. That'll be hard to do, however, for folks who aren't maxing out those accounts.

The bottom line: Saving like mad is the only way you'll retire. Sure, if you've worked for many years for a big company such as General Motors (NYSE:GM) or 3M (NYSE:MMM), your pension and Social Security might see you through. (On the other hand, if you've worked for an airline such as Delta (NYSE:DAL) or American (NYSE:AMR), don't count those pension benefits till they're hatched.) But the factor that will have the most influence on the quality of your retirement is how much you have saved for that sunny day.

So why aren't people stuffing their retirement accounts? A whole bunch of reasons, but my guess is that people think that every dollar saved leaves one less dollar to spend. However, depending on your circumstances, deciding not to save that dollar leaves just 70 cents to spend. Why? Because not saving costs money. It's true. If you have the opportunity to put more money into a tax-friendly account such as a 401(k), a 403(b), or an IRA, yet choose not to, then you are paying for that decision right now. Here's how.

More taxes today
Before money makes it from your employer's bank account to your paycheck, Uncle Sam takes a bite. However, you can prevent that by contributing money to the retirement plan at work or to a deductible traditional IRA (if you're eligible), since your contributions reduce your taxable income dollar for dollar. Put another way, if you're in the 28% tax bracket, every dollar that doesn't go into a retirement account increases your tax bill by 28 cents.

More taxes tomorrow
Capital gains, interest, and dividends on investments in non-retirement accounts get taxed in the year you receive them. Not only does this increase your tax bill year after year, but it leaves less money behind to make more money. However, the money in a 401(k) or an IRA grows tax-deferred, which means you don't pay taxes on that money until you make withdrawals in retirement. (If you have a Roth IRA, you won't ever pay taxes on the growth, though you won't get a deduction on the contributions.)

You don't get the employer match
Picture this: Your boss is holding out $50 to you -- all you have to do is make sure $100 from your next paycheck goes into the company retirement plan. That's right. If your employer offers a 50% match on retirement contributions, he's essentially offering to pay you to save. For every $100 you don't contribute to the plan, you leave $50 of bonus pay in your boss's pocket.

Let's tie all this together in one tidy example to see how utilizing retirement accounts can do more for an investor than improve his post-employment prospects. How much could a hypothetical investor save by contributing to a retirement account?

  • Income-tax savings realized by contributing $6,000 a year to a 401(k) (assuming a 28% tax bracket): $1,680
  • Money gained by receiving the company match (50%): $3,000
  • Income-tax savings from tax-deferred growth (assuming 6% interest on $6,000): $100

Thus, for this fictional fellow, contributing $6,000 to a 401(k) increased his net worth by an additional $4,780 in one year.

Of course, that number would be smaller for folks in lower tax brackets or who don't receive a match on their contributions. On the other hand, this illustration doesn't consider other benefits, such as state income tax savings and more money to compound through the years. Even if someone doesn't receive an employer match, the lower taxes are compelling.

Then there's always that little benefit of actually being able to retire. Saving is the first step. Choosing the right account, the right investments, and the right way to make withdrawals are also important (topics I'll discuss often in my Motley Fool Rule Your Retirement newsletter service). But for now, just contribute as much as you can to your retirement accounts. You can't afford not to.

Robert Brokamp, who doesn't own any of the companies mentioned in this article, is the editor of the Motley Fool Rule Your Retirement newsletter service. Try it free for 30 days. The Motley Fool is investors writing for investors.