One more time: Know when to fold 'em...

— -- Q: I'm a 24-year-old investor and I have a problem: I can't seem to sell a stock. What pointers can you give me?

A: If you think buying a stock at the right time is hard, try selling it at the right time.

Many professional investors have told me that they, too, struggle with selling.

If you sell too early, and the stock keeps going up, you kick yourself for missing potential gains. But if you sell too late, and the stock collapses, you can watch your gains evaporate, or worse, lose money.

Then, there are the tax ramifications of selling. If you dump a winning stock before you've owned it a year or longer, your gains are taxed at your ordinary income tax rate, not the lower capital gains rates. And if you never sell losing stocks, you'll miss out on deducting up to $3,000 a year of your losses from your taxable income, which provides a saving at tax time.

With that said, there are a few rules of thumb when it comes to selling stocks:

• Don't let losses on individual stocks get to 10% or more of what you paid. Individual stocks are extremely volatile, and a good defense is just as valuable as a good offense. Cut your losses quickly. Your goal should be to never let an unrealized gain turn into an unrealized loss.

Again, we're talking only about individual stocks here. With diversified investments, such as stock index mutual funds or exchange-traded funds (ETFs), there's not as much need to worry about short-term swings. You're diversified in a fund, and that reduces your risk.

• Know your sell price when you buy. I can't tell you how many investors buy a stock, hang on and then get paralyzed when the stock drops. Don't make this mistake. When you buy a stock, write down the price you will sell at and stick with it. You can read more on this here.

• Consider using options. For people with serious selling problems, options can be a useful tool. With options, you can do a number of interesting things, such as buy insurance to protect you from catastrophic downsides or lock in a gain. Just make sure you know what you're doing, because, used incorrectly, options can be financially disastrous. You can find a couple of good places to get more information about options here.