Ask yourself this: "What if I'd never sold a stock?"

Would you have more money now, or less? I set out to answer that question myself this morning, and to back it up with some hard data. I chickened out.

I knew the answer. If I had never sold a single share of stock, I would be ... richer than I am today. How much richer? Much richer. I can't give you a precise figure, because I couldn't bear to run the numbers.

It gets worse and worse
I bought Yahoo! on a tip back in January 1997. I dumped it a few months later for a quick double. OK, that's not exactly true. In fact, it's a lie. But the point is I could have, and it illustrates my point.

"I sold Yahoo! back in 1997" is one of the most sickening things you'll ever have to admit to another investor -- yes, even worse than admitting that you suffered a 40% haircut in 2008. After all, since that first double, Yahoo! has doubled again ... many, many times.

I didn't flip the Google (NASDAQ:GOOG) IPO for a quick double, either. But I know a little how that feels. Pull up a long-term chart for medical-waste handler Stericycle (NASDAQ:SRCL). You'll see a steady ramp upward, connecting $2 to the top of the freakin' world. (OK, only to the mid $60s, but you get my point.)

You guessed it. I bought Stericycle for a few bucks in 1997 and sold it the next year after a tidy profit. Now it's somewhere in the mid-$40s. That's what I call the most painful double of my career.

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

No, I sold my multibagger to book a nice gain. But what did I really "book"? Zip. You never do, unless you pull your profits 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 this market. A lot of folks claim to do it -- and a few actually seem to pull it off for a time -- but not me. In fact, you might want to brace yourself, because I'm going to go one giant step further than that.

I barely believe in valuation
At least not when it comes to selling. Sometimes a stock gets so cheap you have to buy it. Here at the Fool, folks got downright giddy when Apple (NASDAQ:AAPL) and Cisco (NASDAQ:CSCO) plunged in early 2003. That worked out.

Now the value guys are finally poking around the financials. The winners (OK, the survivors) like US Bancorp (NYSE:USB) and Wells Fargo (NYSE:WFC) are consistently among the heavy traders. It's a high-stakes game, but I've been a buyer myself. We'll see how that works out.

Either way, valuation is well and good if you've got money to put to work, but the math gets dicey when it comes to selling -- especially growth stocks, and especially big winners. The fact is, 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
Let me put it another way. I'm not surprised that my pal Seth Jayson and his crew at Motley Fool Hidden Gems are sitting on 10 recommendations that are still up more than 100% -- even after the recent market pullback.

They work hard, stick to the fundamentals, and understand value. Plus, they're fishing a rich pond. Wall Street isn't snooping around these smaller stocks yet, which creates inefficiencies and pent-up demand.

But just so you don't write me off as a Hidden Gems cheerleader, I'll let you in on a secret: I use the service to lead me to undervalued small caps with big potential. When they tell us to sell, 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
"Market timers" tell you that buy-and-holders like us get wiped out in bad markets. Yet when you pull up long-term charts of "boring" widow and orphan stocks like Procter & Gamble (NYSE:PG), what do you see? A slope skyward. So how on earth does anybody lose money on a stock like that?

Know what else looks like that? The Dow, or the S&P 500, for that matter -- aka the market. Granted, when you zoom in, the ride looks bumpier, but the trend is up. So how do you lose money in the market? Well, you either buy at the top in 2000 -- and only at the top in 2000 -- or you get cute and buy and sell along the way.

Consider this approach instead: Sell your winners when you want to buy a house, furniture, or other major purchase. Sell when you have too much in stocks and you want to buy some bonds, gold bars, or Dickensian village collectibles. Sell when you have too much in any one stock. But sell a stock on valuation alone at your own peril.

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

And when these guys tell you to buy, you'll want to listen -- especially right now when the market is littered with bargains. The folks at The Motley Fool are so confident that this is true, they are investing $250,000 of real money in stocks from the Hidden Gems scorecard. You can follow along in real time if you like.

Simply accept a free 30-day trial to Hidden Gems. Within five minutes, you can verify everything I've just told you and check out the real-money portfolio without risking a cent. If you don't like what you see, don't pay.

But whatever you decide, just promise me you won't get too cute. For a peek at the Hidden Gems scorecard, including the team's top five picks for new money now, and to find out more about your special free trial, click here.

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

Fool writer Paul Elliott promises to keep you posted on the progress at Motley Fool Hidden Gems. Paul doesn't own shares of any companies mentioned. Google is a Rule Breakers selection. Apple is a Stock Advisor pick. Procter & Gamble is an Income Investor recommendation and a Fool holding. The Motley Fool is investors writing for investors.