Failing to contribute enough to receive a maximum employer match. That is free money, providing an immediate and risk-free, tax-deferred return on your savings.

Borrowing from your 401(k) when you do not have to. Is a kitchen remodeling worth jeopardizing your future?

Trying to time the market. Do not jump from one investment to another, chasing "hot" sectors. Even pros cannot consistently time the market successfully.

Being too conservative. If you have 10 or more years until retirement, do not avoid stocks. Over long periods, they have outperformed bonds and other alternatives.

Being too aggressive. Consider keeping money you will need within five or so years out of stocks.

Holding too much of your employer's stock. Try to have no more than 10 percent to 20 percent of your plan assets in any one company. Even respected companies can implode, with disastrous results for employees.

Failing to allocate or rebalance. Decide what percentage of your money you want in stocks, bonds and cash-like investments, according to your age, risk temperament and goals. Then monitor and rebalance your holdings every year or so.

Keeping a default election that automatically invests your money in an ultraconservative option. This can doom you to low returns.

Cashing out after a job change. Too many people do this each time they switch jobs, leaving themselves with little to retire on. Leave your money in the plan, or roll the balance into your new employer's plan or an IRA.

Ignoring index funds. Your best bet for stock investments in a 401(k) is usually an index fund, such as one based on the S&P 500 or the total stock market. If your plan does not offer one, ask about it.

Above all, do not fail to participate in your plan in the first place. Learn more about 401(k)s at www.fool.com/money/401k and www.401khelpcenter.com/

Employee_index.html.