NEW YORK (CNN/Money) - If you want to retire comfortably, there are four key financial changes you're going to have to adapt to.

Today's and future generations of retirees aren't going to get anything remotely approaching the kind of windfall Social Security recipients received in years past.

Get to know Oz • Bye-bye Kansas • Bye-bye pension; hello 401(k) • Living longer; working longer Plan your exit strategy • Don't outlive your money • Returns: Don't assume too much • The ABCs of annuities • Set up an annuity solution

For one thing, there aren't enough workers paying into the system to provide benefits comparable to those past generations received.

Given the uncertain outlook for this program, some financial planners suggest expecting nothing. I think that's a little extreme. Even if the social Security trustees' projections are accurate and the trust fund runs dry in 2042 or so, it's not as if the Social Security system will go bankrupt then. Payroll and income taxes will continue to flow into the system.

But one way or another I expect that future Social Security benefits will be smaller than they've been in the past.

If you are relying primarily on Social Security to carry you through retirement, you are either: a) counting on a very short retirement, b) counting on a very grim retirement, c) fooling yourself, or d) all of the above.

Suffice it to say that planning to make Social Security the cornerstone of your retirement isn't really planning at all.

Traditional corporate, or defined-benefit, pensions are the types of pensions most of us think of (or used to think of) when we hear the term pension -- that is, one in which the company puts money into an investment fund and, regardless of the performance of the investments, promises to pay you a monthly check for life based on how many years you worked at the company and the size of your salary.

But as these types of pension plans were nearing their peak in the late 20th century, the seeds for their demise was being sewn.

Companies began to realize that with this type of pension they could be on the hook for much bigger liabilities than they'd expected, so many firms decided they were better off shutting down their defined-benefit plan, or at least not starting any new ones.

Fortunately, even as traditional defined-benefit pension plans have been disappearing, most of us have had access to a growing array of other types of retirement savings plans.

At the top of the list are defined-contribution plans such as 401(k)s that allow you to contribute a percentage of your salary before taxes into a variety of investments, typically mutual funds. In many cases, employers will match a portion of what you put into the plan.

But 401(k)s and similar plans require you to play a much bigger role in planning for retirement in two crucial ways:

First you've got to take the initiative to put your money into these plans. If you contribute only a small percentage of your salary to your 401(k) plan, then you will only have a little bit of money at work for your retirement. If you don't contribute any of your salary, then the plan is absolutely no help to you at all.

Second, you've got to assume responsibility for investing whatever you save. In the old company-funded defined contribution plans, the company hired professional investment advisers to invest the plan's assets. But in the world of defined contribution plans and IRAs, you are the investment manager. You've got to decide which how much of your money should be in bonds and what kind of bonds. You've got to figure out how much to put in stocks and what kind of stocks. Which brings us to the final financial change...

Back during the late great bull market of the 1990s, we came to believe you could earn high returns without risk. All gain, no pain!

This attitude in turn gave us the erroneous sense that saving isn't the key to achieving a secure retirement, smart investing is.

Let's say, you're 40 years old and want to accumulate $500,000 by age 65. Well, if you could count on annual returns of, say, 15 percent year after year, then putting away just $180 a month would get you to your goal. (For simplicity's sake, I'm ignoring taxes here.)

But what if it turns out that 15 percent is unrealistic and that you really shouldn't be counting on more than, say, an 8 percent return on a regular basis?

Then to have a shot at accumulating $500,000 by the time you hit 65, you would have to put away more like $550 a month -- or three times as much.

Neither I nor anyone else knows for sure what returns the financial markets will deliver in the years ahead.

But I think it's become pretty clear now that the blimpish returns of the late 1990s were an anomaly, little more than a crazy outgrowth of the whole dotcom-New-Era-irrational-exuberance phase.

It would be flat out irresponsible to base one's retirement planning on those kinds of returns. I think most people are resigned to that reality.

But what I don't think has necessarily sunk in is the fact that lower returns mean we've got to save more money to reach the same retirement goals.