During the bear market, you heard plenty of advice telling you not to think about selling your investments and changing your retirement saving strategy. After all, they argued, with the markets down, panic-selling could mean taking a big loss -- one that could remain permanent if stocks rebounded before you managed to reinvest the proceeds.

That advice turned out to be right. But as an investor, you still need to remain vigilant. In particular, if you've turned over the duties of picking stocks to a mutual fund manager, you have to understand that you won't always pick the best funds. And even if you do manage to find a great fund, it won't always stay great forever.

How will I know?
The same factors that help you choose a good actively managed mutual fund can help you decide when it's time to get out. When scouting out funds for your portfolio, you should look for reasonable costs, experienced fund leadership, consistent outperformance, and a management team that's willing to put its own money into the funds it oversees.

Conversely, if you've already invested in a fund and one or more of these favorable characteristics disappears, be wary. When extraordinary managers leave the fund, their successors aren't guaranteed to maintain the fund's winning ways. Several years of underperformance can mean that the fund's management team has lost its edge. Rising costs can indicate internal problems with a fund company -- or, if accompanied by outflows of capital, they can suggest a loss of efficiency.

Walking in a legend's shoes
As an example, consider Fidelity Magellan, the flagship mutual fund formerly run by fund superstar Peter Lynch. Lynch's "buy what you know" philosophy led him to buy companies like Yum! Brands' (NYSE:YUM) Taco Bell unit, Apple (NASDAQ:AAPL), and Automatic Data Processing (NASDAQ:ADP) at opportune moments, and Magellan earned nearly 30% annually during his 13-year tenure. The next two managers that followed Lynch also managed to outperform the S&P 500 index, albeit not as spectacularly.

For Robert Stansky, however, the story would be different. Taking over Magellan in 1996, Stansky was the first Magellan manager who failed to match the benchmark index. One criticism that Stansky faced was being a "closet indexer" -- managing the fund so that its return would never be much different from the S&P 500.

But Stansky also made some bad calls. In the fund's 2002 semiannual report, Stansky admitted to having underweighted consumer-staples stocks like Procter & Gamble (NYSE:PG) in favor of Best Buy (NYSE:BBY), Home Depot (NYSE:HD), and Tyco International (NYSE:TYC). The latter three companies lost more than half their value during the bear market from 2000 to 2002, while consumer staples stocks performed reasonably well. As a result, many of Magellan's shareholders jumped ship.

When things change
As a fund investor, you have to be on the lookout for changes within your funds. Some events, like Lynch's departure from Magellan, are so obvious that you can hardly miss them. But changes don't have to be as conspicuous as a superstar's exit to warrant casting a more critical eye toward your fund choices.

During good times, it's easier to give fund managers some leeway -- even if you're not outperforming the market, you're still usually making money. But when times get tough, it's more important to know when you could do better somewhere else. If your mutual fund stops performing for you, don't hesitate to find another that will make you more money.

Get the latest mutual fund advice for your retirement portfolio from Foolish fund expert Amanda Kish. In her most recent article, Amanda talks about three lessons you should learn from the Dubai crisis.

This article was originally published on Mar. 4, 2008. It has been updated by Dan Caplinger, who doesn't own shares of any companies mentioned. Apple and Best Buy are Motley Fool Stock Advisor recommendations. Best Buy and Home Depot are Motley Fool Inside Value picks. Automatic Data Processing and Procter & Gamble are Motley Fool Income Investor recommendations. The Fool owns shares of Procter & Gamble and Best Buy. Try any of our Foolish newsletters today, free for 30 days. The Fool has a disclosure policy.