When parents think about how to invest for their children's college education, they naturally look to retirement planning as a model. Don't.

It's a dangerous mistake to think of 529 college savings plan as "mini 401(k)s," cautions G. M. "Buz" Livingston III, a certified financial planner with Livingston Financial Network in Santa Rosa Beach, Fla.

Workers saving for retirement can tolerate the volatility of the stock market, because they usually have decades in which to build up savings. And once they retire, they don't necessarily have to convert their entire nest egg to cash. They might live another 20 to 30 years and need the growth that equities provide.

Unlike retirement payouts, says Livingston, "the payout period for a 529 plan is short: four or five years."

To ensure the money is there when needed and not slashed by a 30% market downturn, families should start moving college investments from stocks and bonds to cash three years before the start of college.

It can be difficult to make the switch to cash if investments are performing well, says Peg Eddy, certified financial planner and president of Creative Capital Management in San Diego. She knows, because she has been there.

Eddy and her husband and business partner, Bob, saved for college for their two sons, now 26 and 23, from the time they were born.

College 529 plans weren't available then, but the couple made do with monthly taxable investments into the same mutual funds they were using for retirement -- two growth funds and one balanced fund.

As each son reached his freshman year, Eddy cashed out one-third, then the second third by the sophomore year and the remainder by the junior year. It was the mid-1990s. The stock market was going gangbusters, and so were the funds.

Eddy hated pulling the money out of the funds, because she could see her own retirement investments continuing to soar. "It killed me," she says. "But I knew those bills were going to come. You've got to take money off the table."

Their discipline paid off, financing two private undergraduate college degrees, one from Washington and Lee University in Lexington, Va., for older son Sean and the other from Santa Clara University in Santa Clara, Calif., for younger son Ryan, with no debt.

While some families start college savings plans when their children are born, others don't get going until the children are 10 or 12 years older. That's too late to enable them to safely weather the ups and downs of the stock market.

The past five years provide a case in point. While the

S&P 500

rewarded investors by quadrupling in value between 1990 and 2000, those who contributed money in 2000 have seen the benchmark index lose about 19 1/2% of its value, despite a strong 2003 performance.

Livingston notes that investing in bond funds at these ages carries risk as well, because they can lose value if interest rates rise, as has been the case recently. "Bond funds are not fixed-income," he says.

Given that the average 529 savings plan holds just $7,000, Livingston asks, "Should that money be put at risk?" Definitely not.