Over the years, we've heard from a heartbreaking number of people who have put their retirements at risk by making common but costly mistakes with their 401(k)s. We've discovered five key mistakes people make with their 401(k)s.

We'll help you avoid them—and if you're already making one of them, don't panic! It's never too late to change what you're doing and get your 401(k) back on track—and we can help there, too!

Mistake #1: giving up free money

If you are not participating in your company's 401(k) program—even if you think you are too close to retirement for a 401(k) to do any good—you should be.

Every little bit you save now counts, and there's also the tax benefit to consider. Remember, you defer income taxes on money you put into your 401(k) until you actually withdraw it.

Now, for all of you patting yourselves on the back and saying "I participate in my company's 401(k)," congratulations! But tell us, are you taking advantage of your employer's matching program? Most companies offer to match your funds up to a certain amount. That is the closest you are ever going to get to free money!

If your company matches 50% of your contributions up to 6% of your income, the smartest thing you can do is contribute at least 6%. Your company's benefits manager should be able to help you figure out what percentage you have to contribute to get the full employer match.

Mistake #2: not swinging for the fences

Making sure you get all of your employer matching funds is a good start, but you don't have to stop there! Your 401(k) limits are determined by your plan's cap, your salary and the $15,500 annual maximum allowed by law. Depending on your plan, you may be able to contribute above the employer match rate. If you are over 50, you may qualify to make additional "catch-up" contributions of $5,000 per year.

Why go above the employer match percentage? Take a look at the math.

If you contribute $200/month for 30 years...

...with a 10% annual return (the average market return since 1900), you'll have $452,000. Change that to $400/month, and you get $900,000! How do you like that kind of money?

Find out what your maximum is and get as close to it as you can. Use this 401(k) calculator on Dolans.com to see how much money your 401(k) could provide for your retirement.

Mistake #3: taking the easy way out

One of the easiest mistakes to make is to throw your money into a guaranteed return fund and leave it there. Yes, a "guaranteed return" feels safe, but if you put all your money in it, you're not investing for growth. 401(k)s have many more options to offer people now, including lifestyle and target retirement funds.

Taking the time to do a little research could make a big difference in the long run.

Mistake #4: the one that really makes us nuts

We get people calling the radio show all the time saying "Hey, I want a new car. How about I take a loan from my 401(k)?" Our answer is no, no, a thousand times, no! Do NOT take loans from your 401(k) unless it is an absolute dire emergency. If you're drowning in medical bills or facing foreclosure, then yes, tap your 401(k). But otherwise—leave that money where it is!

When you pull money out of your 401(k), you're stopping tax-deferred growth in its tracks. And while your 401(k) may seem like a convenient source for cash, if you're fired, laid off, or change jobs, you have to pay that money back immediately or get taxed on it as income. Plus pay a penalty on top of that if you're under 59½ ! How convenient does that sound? For more on 401(k) loans, check out this video at Dolans.com.

Mistake #5: cashing out early

You've changed jobs, and there's this big check just sitting ther tantalizing you. You figure, what the heck? I'll take the tax hit, get the cash and go buy some stuff. Say no to this impulse! You're going to want that money later. But don't just leave that money languishing at your old employer's, forgotten and neglected.

If you can't roll it in to a new plan immediately, set up an interim IRA, and roll it in to the new 401(k) once you are eligible. Do make sure it is a separate IRA, or you'll have one heck of a paperwork nightmare.

Now more than ever, you have to make smart decisions about your money. We can help. We're Ken and Daria Dolan, and we've spent over 20 years helping people just like you find real money solutions, build wealth and live well. Come join us at our new Straight Talk on Your Money service, and start making smarter, more confident and successful choices with your money, every day of your life.