Lighten up ... rotate out ... take some profits.

Call it what you will, but it means "selling." And it's tricky business. So before you reach for the rip cord, ask yourself this:

What if you had never sold a stock?
Honestly, would you have more money now, or less? I set out to answer that question for myself this morning, and to back it up with some hard data. I chickened out.

Heck, 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 knew that once I saw it for myself, I would scream. How about you?

It gets worse, and worse, and worse
I bought Microsoft (NASDAQ:MSFT) back in 1987. I sold it the next year for a quick double. OK, that's not exactly true. In fact, it's an out-and-out lie. It is, however, one the most sickening things I can imagine having to say as an investor.

I didn't buy and sell Dell (NASDAQ:DELL) as a small cap, either. But I know how it feels. Pull up a five-year chart for Pulte Homes (NYSE:PHM) and you'll see a steady ramp upward, connecting $10 to the ... top of the freakin' world. (OK, only to around $100, but still.)

You guessed it, I bought Pulte at around $10 and sold it a year or so later for around $16. Now it's in the mid-$90s. That, my friend, is what I call the most painful 60% profit of my career.

"So what did you do with the caaash?"
How should I know? I probably bought another stock, though do you think it did as well as Pulte? I know for a fact that I didn't have a better stock in mind when I sold it -- and that I didn't buy a house or even furniture (you'll see in a moment how this is relevant, believe it or not).

I sold my meal ticket for no other reason than to lock in a nice gain. But what did I really "lock in"? Zip. You never do unless you pull that money right out of the market -- which is not something I think you should seriously consider, especially if you're in your prime investing years.

That's right. 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 it's not for 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. Yes, there are times when a stock gets so cheap you simply have to buy it. IBM (NYSE:IBM) in 1994 or McDonald's (NYSE:MCD) in early 2003 come to mind. But the math gets more dicey when it comes to selling -- especially growth stocks, and especially winners.

How about a funny example? Two years back I asked Tom Gardner -- we'll talk more about Tom and his Motley Fool Hidden Gems in a bit -- for the one stock I should buy for my Roth IRA. "I love Moody's," Tom replied, "but it's a little pricey at these levels." I bought it that instant (it's up about 75% since).

What was I thinking? It's simple: I'll take a company a great investor absolutely loves over a "bargain" any day. If that same investor tells me the stock is "a bit pricey," I love it that much more. The fact is, I've met some great stock pickers in my day, but only a very few great sellers. Come to think of it, I've never met a great seller.

Promise me you won't get too cute
I'm not the least bit surprised that Tom Gardner and his Hidden Gems crew have picked half a dozen stocks that have more than doubled in value over the past two years. They work hard and stick to the fundamentals. Plus, they are fishing a rich pond. Wall Street isn't snooping around yet, which creates inefficiencies and pent-up demand.

But I wouldn't want you to think I'm a Hidden Gems cheerleader, so I'll tell you a secret: I only use the service to lead me to undervalued small caps with big potential. Sure, from time to time Tom will tell his folks when to sell, but I typically don't listen -- and probably won't in the future. Not unless the story is really broken. And certainly not if it's a winner. I will never sell on valuation.

That's how tragedies happen
One day you'll find yourself chatting with a stock jock, and the fellow will say something like, "Yeah, but a lot of people lost money over the years on Home Depot (NYSE:HD) or Wal-Mart (NYSE:WMT)". But then you will look at a long-term chart, and it's a gentle slope skyward. So how on earth did anybody ever lose money on those stocks?

Know what else looks like that? The S&P 500 -- a.k.a., the market. Granted, when you zoom in the ride looks a bit bumpier than it appears on a 30-year graph, but the long-term trend upward is unavoidable. So, how do you lose money in the market? Well, either by buying at the top in 2000 -- and buying only at the top in 2000 -- or by getting cute and buying and selling along the way.

Consider this approach instead: Sell your stocks when you want to buy a house or furniture. Sell when you have too much in stocks and you want to buy some bonds, gold, or collectibles. Sell if you have too much in any one stock. But sell a stock, or the market, on valuation at you own peril.

The little picture
Like I said, when you join a service like Hidden Gems, smarter investors than I will tell you when to lock in your gains. The great thing is that the choice is yours. However, when Tom and the Hidden Gems gang tell you to buy, you sure as heck want to listen. After all, as of July 22, the stocks recommended in Hidden Gems are up 35.7% on average. That's compared to about 10% for the S&P 500.

Are you earning returns like that? If not, you're in luck. Tom is offering a special free trial to Hidden Gems right now. Take him up on it, and you can check out everything I've told you without it costing you a cent. Whatever you decide, just promise you won't get too cute. Click here to find out more.

Fool writer Paul Elliott promises to keep you posted on Tom Gardner's progress at Motley Fool Hidden Gems . All picks and results are posted on the Hidden Gems website. Paul does not own shares of any company mentioned in this article. Dell is a Motley Fool Stock Advisor recommendation. Home Depot is a Motley Fool Inside Value recommendation. The Motley Fool isinvestors writing for investors.