Lighten up ... rotate out ... take a little off the table.

Whatever you call it, it means selling. And no matter what the Fast Money guys tell you, selling is tricky business. So before you buy the market-timing mantra and pull the ripcord, ask yourself this:

"What if I had never sold a stock?"
Would you have more money now, or less? Inspired by the buy-and-hold debate simmering in Fooldom, I set out to answer that question this morning, and to back it up with some hard data.

Then I chickened out.

I already knew the answer. If I had never sold a single share, I would be ... richer than I am today. How much richer? Much richer. I can't give you a precise figure, because I knew that once I saw it for myself, I'd have to go home and kick the dog.

How about a quick example?
I bought Qualcomm (Nasdaq: QCOM) on a tip back in September 1992. I dumped it that Thanksgiving for a quick double. OK, that's not exactly true. In fact, it's an out-and-out lie. But I could have, and it does illustrate my point.

"I sold Qualcomm in November 1992" is one of the most sickening things you'll ever have to admit to a fellow investor. After all, since that first double, Qualcomm has doubled again ... many times. I didn't flip rival Nokia (NYSE: NOK) for a quick double, either. But I know a little how that feels.

If you were to pull up a long-term chart for medical waste handler Stericycle, you'd see a steady ramp upward, connecting $2 to ... the top of the freakin' world. (OK, only to around $60, but still.) You guessed it: I bought Stericycle in the single digits in 1998 and sold it the next year. That, my friend, is the most painful double of my career.

"So what did you do with the cash?"
How should I know? I probably bought another stock, but do you think it did as well as that? Fat chance. I know I didn't have a better stock in mind when I sold it. I don't recall buying a house with the proceeds, or even furniture. (You'll see how this is relevant in a moment, believe it or not.)

No, I dumped a future 25-bagger to lock in a double. But what did I really "lock in?" Zip. You never do, unless you pull that money straight out of the market, which is not something you should consider now, especially if you're in your prime investing years.

That's right. Tempting as it is, I don't think you should try to time the market. A lot of folks claim to do it -- and a few actually seem to pull it off -- but not me. In fact, you might want to brace yourself, because I'm going to go one giant step further.

I can barely make sense of valuation
At least, when it comes to selling my winners. Sometimes stocks get so attractive you have to buy. For example, when the threat of litigation knocked Altria (NYSE: MO) to ridiculous lows in 2000. Or when cheap oil did the same to Halliburton (NYSE: HAL) a few years later.

Or right now, when tech giants such as IBM (NYSE: IBM) and Microsoft (Nasdaq: MSFT) are trading at around 12 times earnings. Buying value makes sense. But I’d argue that the math gets dicey when it comes to selling -- especially selling your winners.

I've met some great stock pickers in my day, but not many great sellers. Come to think of it, I've never met a great seller.

Promise me you won't get too cute
That's why I wasn't surprised to hear that my colleagues Seth Jayson and Andy Cross are instructing their Motley Fool Hidden Gems subscribers to follow Warren Buffett and legendary fund manager Marty Whitman’s lead and buy this market. They work hard and stick to the fundamentals.

Plus, as dedicated small-cap value investors, they're fishing a rich pond. As a rule, Wall Street isn't snooping around these lesser-known stocks yet, which creates inefficiencies and pent-up demand, as I learned the hard way myself with Stericycle in 1998.

But just so you don't write me off as a Hidden Gems cheerleader, I'll let you in on a secret: I use these guys to lead me to undervalued small caps with big potential. From time to time, they tell me to sell, but I typically don't listen -- and I probably won't in the future. Especially not if it's a winner. I never sell on valuation.

That's how tragedies happen
After all, market-timers tell you that buy-and-holders like us get wiped out in bad markets. (You’ll hear more of this in the next couple of weeks.) But then you pull up chart after chart of "boring" old stalwarts. For example, Dow component United Technologies (NYSE: UTX), another stock I sold for a “gain.”

What do you see? A gentle slope skyward, that's what. So how on Earth did anybody ever lose money buying and holding stocks like that? Good question. Know what else has a long-term chart that looks like that?

The Dow. Or the S&P 500, for that matter -- a.k.a. the market. Granted, when you zoom in on periods like this one, the ride looks bumpier, but the long-term trend is skyward. So how do you lose money in stocks? Well, you either buy at the top -- and only at the top -- or you get cute and buy and sell along the way.

Consider this approach instead: Sell your stocks when you want to buy a house, furniture, or other major purchase. Sell a little when you find you have too much in stocks and want to buy some bonds. Sell when you have too much in any one stock. But sell a stock, or a dicey market like this one, on valuation alone at your own peril.

You don't have to go it alone
OK. Enough preaching. Like I said, when you're a member of a stock-picking advisory service like Hidden Gems, much smarter investors than I will tell you when to lock in your gains. But remember, the choice to sell is always yours.

And right now, I'm not selling. I'm buying. If you'd like to join me, but are a little gun-shy or short on ideas, how about this? Take a free 30-day trial to Hidden Gems right now.  As we speak, the management team is putting $250,000 in real money to work in a portfolio of small caps.

So far, so good. Seven of the eight stocks they've bought since launching the portfolio in March are beating the market -- including gains of 30% and 130%. You can check out all their results, along with every past pick and all back issues, right now. It'll take you five minutes, tops.

 Best of all, the complete service is free for a whole month, and there's no pressure to subscribe. But whatever you decide, just promise me you won't get too cute. To see how easy it is to accept your free trial, click here.

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This article was originally published on July 22, 2005. It has been updated.

Fool writer Paul Elliott promises to keep you posted on the progress at Motley Fool Hidden Gems. All picks and results are posted on the Hidden Gems website, which is all yours with a free trial. Paul doesn't own any stocks mentioned. Nokia and Microsoft are Motley Fool Inside Value recommendations. The Motley Fool is investors writing for investors.