You can't anticipate at age 25 what your final salary will be at retirement -- and therefore you can't know what 85 percent of your ending salary will be (that's the benchmark retirement analysts generally say you should set to maintain your standard of living once you're out of the workforce). So most retirement planners recommend that you save 10 percent to 15 percent of your salary in a retirement account, starting from day one. Keep in mind that contributions to a traditional 401(k) or IRA are in pretax dollars. Contributions to a Roth 401(k) or Roth IRA account are made with after-tax dollars, but withdrawals in retirement are tax-free, assuming the account has been open for at least five years.

One big advantage to starting early is that it lets compounding work longer on your behalf. Say you're 25, earning $40,000 a year, and you initially set aside 6 percent of your salary, building to 12 percent over the next six years. If you save 12 percent for the rest of your career, you'll have amassed $639,236, or 8.7 times your final salary. (The example, from Fidelity, assumes that your investments earn an average annual return of 5.5 percent, your company kicks in 3 percent a year, and you have a final salary of $73,640.) If you wait until age 30 to start saving and follow the same scenario, you'll end up with about $100,000 less.

If you're well into midlife and never got the memo to save early and often, "worry about how long to work rather than how much you save," says Alicia Munnell, of the Center for Retirement Research. By working even a few years extra, you not only keep money coming in and your savings growing, but you also reduce the number of years in which you need to tap your nest egg. More important, Social Security benefits increase by about 8 percent a year for each year you delay claiming benefits between age 62 and 70. For example, if you start saving at 45, you'd have to set aside a whopping two-thirds of your salary annually to get to a sum that replaces 80 percent of your income at a retirement age of 62, according to the Center for Retirement Research. Start at 45 and retire at 70 and you'd have to save only 18 percent -- still stiff but more doable.

(Jane Bennett Clark is a senior editor at Kiplinger's Personal Finance magazine. Send your questions and comments to moneypower@kiplinger.com. And for more on this and similar money topics, visit Kiplinger.com. Kiplinger's has a service to pinpoint the ideal time to claim Social Security to maximize benefits. Visit http://kiplinger.socialsecuritysolutions.com.)