Trying to beat the market

Timing the market would require a "Back to the Future"-style time machine. That's why investing a little bit at a time, regardless of the market's behavior, is the safest way to go. Retirement accounts such as 401k's, which invest money from your paycheck each month, make it easy to invest this way.

Paying too much attention to the Dow

Focusing too much on the ups and downs of the stock market just causes stress. When the market's plunging, concentrate instead on your hobbies, family and getting outside. Avoid cable television news, which often treats every dip in the market like a major crash. If your investments are well-diversified, then you've done all you can.

Counting on Social Security

As they think about retirement, today's 30-somethings should be aware that the Social Security trust fund is estimated to run negative in 2037. That means that if nothing changes, benefits will shrink to about three-quarters of what they are now, because only money that is being paid into the system will be paid out. Young professionals need to plan on funding the bulk of their retirement with their own savings.

Overspending on gifts

Click here to become a fan of MSN Money on Facebook

Pollster John Zogby has found that the amount of money people say they intend to spend on Christmas gifts has been steadily declining since 2001. Consider joining that movement by making your gifts more meaningful and less expensive. Instead of pricey jewelry and electronics, consider cookbooks and museum dates. You can also consult websites such as to find unique do-it-yourself gift ideas.

Underestimating tax bills

People who earn money beyond their usual paycheck, from freelance work or a side business, are most at risk for owing a lot of money in April. And poor tax planning can also trigger additional fees. Married couples who earn similar high salaries are also at risk, because of the so-called marriage penalty. Check to see if you've been paying roughly the correct amount of taxes by reviewing your payroll stubs or other documentation.