We spend a lot of time at the Fool writing about best practices in the world of stock and mutual fund investing. What we need more of, however, is advice about what not to do as an investor. Similar to the sporting world, where sometimes the best offense is a good defense -- avoiding mistakes is just as important to your financial health as making good investments.

In that spirit, I submit to you seven mistakes that, if you don't avoid them, can cost you tens of thousands of dollars (or more) over the decades. This list is courtesy of Fool fund analyst Shannon Zimmerman -- whose Motley Fool Champion Funds newsletter service is beating its benchmarks by 19% versus 8% since its inception nearly two years ago.

Shannon's don'ts:

1. Don't pick the funds for your 401(k) on your first day of work and never look back. Hey, things change. Among them: your investing knowledge, your investing philosophy, the fees of the funds you selected, and the funds available to you in your 401(k). Check your options at least annually, or you may one day discover you should have switched funds years ago!

2. Don't ignore costs, fees, and percentages just because the numbers seem so small. Low-cost index funds have expense ratios well below 0.2%. Among actively managed funds, you should be on the lookout for long-haul market-beaters that will ding you 1% or less. Anything above these two numbers should cause you to give you your fund a long hard look. Why? Let's say the market averages 10% per year over the next 30 years. An investor contributing $4,000 annually to an index fund with a 0.1% expense ratio (the Fidelity Spartan 500 (FSMKX), for instance) will earn $53,934 more than one in a fund charging 0.5% (Dreyfus S&P 500 Index (PEOPX), for example). That, my friends, is real money!

3. Don't give too little -- or too much -- attention to tax consequences. Just as fees can have a dramatic effect on your future wealth, so can taxes. But while you'll want to follow tax-efficient strategies, you can't let taxes steer all of your investing decisions. At the end of the day, after all, it's how fat a fund's after-tax return is that matters, not how miserly the manager has been with Uncle Sam.

4. Don't check your funds' tickers more often than you check your mirror. Short-term swings in stock and fund prices are usually just noise, and watching every movement can drive you to some poor decision-making. For a truer picture of your funds' performance, check their one-, three-, and five-year numbers, and remember: There's nothing inherently magical about a mutual fund. It's the manager who matters, and it's his or her track record that you should scrutinize most closely.

5. Don't keep the same asset allocation forever. Your allocation needs will change over time along with your life circumstances. A heavy weighting in the Fidelity Capital Appreciation Fund (FDCAX) will include some high-risk/high-potential-reward holdings such as Genentech (NYSE:DNA), Nokia (NYSE:NOK), Teradyne (NYSE:TER), and Symantec (NASDAQ:SYMC). These more volatile stocks may be fine for aggressive investors with a long-term horizon. But a recent large-cap Champion Funds recommendation is a better choice for conservative investors with more stable holdings like ConocoPhillips (NYSE:COP), Occidental Petroleum (NYSE:OXY), and Wells Fargo (NYSE:WFC).

Bond funds are also an important part of the mix. Shannon has model portfolios for aggressive, moderate, and conservative investors on the Champion Funds website.

6. Don't base your buy and sell choices only on a fund's past quarter. This is still one of the worst mistakes investors make -- chasing last quarter's or last year's success stories. Studies have shown that putting money into the previous year's top-performing fund is a sure path to mediocrity.

7. Don't ignore mutual funds. You can't, really. Even if you prefer buying individual stocks, you probably have a good portion of your nest egg in a mutual fund through your 401(k).

As you can see, it pays to have a good handle on the mutual fund world. Shannon takes you even deeper in Champion Funds with monthly fund recommendations, model portfolios, and even funds to avoid in his "Dud of the Month" column. Check out all his recommendations with a no-risk, 30-day free trial.

Rex Moore has watched Texas' Vince Young score the winning touchdown against USC 24 times on his TiVo. He owns no companies mentioned in this article. Fidelity Capital Appreciation is a Champion Funds selection. Rex tells you all of this because of the Fool's strict disclosure policy.