10 secrets to successfully save for retirement

Never cash out a 401(k) when you change jobs

About half of all workers cash out some or all of their 401(k) when they change jobs. They not only have to pay taxes on the money, but they have to pay a 10% penalty to the IRS.

This is the single biggest wealth-killing mistake many workers will make in their entire careers.

If you have $10,000 in your 401(k) that you've been building up for years and just cash out when you leave your company, you might be lucky to walk away with $7,500. To preserve all of your money, you need to roll it over into an IRA or a 401(k) plan at a new employer to have any chance of saving that first $100,000.

"There are obvious and expensive tax reasons not to cash out a 401(k)," says John Piershale, wealth adviser at Piershale Financial Group in Crystal Lake, Ill. "The purpose of it is for your retirement goals."