I read this article today, and he invited comments.  Here are my comments (his words are in italic):

While no investment approach is successful all of the time, here are six common investing mistakes to avoid:

Inability to take a loss and move on.

This is a good point, subject to what I will mention later.  If you learn new data about a company, such that you conclude it is worth far less than your original estimates, yes, it is likely you should sell.

But often when investors sell after a disappointment, they sell too cheaply.  Bounces often come after disappointments.  The key question is to estimate the new value, and calculate a new implied return, and compare that against the implied returns of alternative stocks.  Are you holding the stocks with the best set of likely returns?

Not selling winners.

The stock may have been a winner, but that doesn’t mean it can’t win more.  Don’t look through the rear-view mirror.  Look through the windshield.  What is your estimate of value NOW?  It may be a lot more valuable than when you first purchased it.  If uncertain, sell a little bit of it – it helps psychologically to do that, because taking a gain will make you more comfortable about the remainder of the position.

But if the stock is one of your leading ideas, even after a run-up, why sell any of it?  Again, rank the idea against the alternatives that you might reinvest in, and choose the idea that gives you the best likely returns, adjusted for risk.

In my opinion, too many people trim winners that have more to run.  Be bloodless, and evaluate the future prospects of the company versus those of alternatives.

Not setting price targets.

Fixed price targets are foolish.  Price targets should be dynamic, and shift with the estimated value of the firm.  Further, evaluate companies against alternative investments.  Only sell the stock of a company when you have a company significantly better in terms of implied returns to replace it with.

Trying to time the market.

I agree that it is difficult to time the market.  That doesn’t mean that it is not worth trying to do it on an intermediate-term basis.  Follow the credit cycle in the corporate bond market, and you will have a good idea of where stocks are likely to go.  When corporate lending falls apart, so do stocks.  Also, momentum tends to persist, so be more aggressive when stocks are above their 200-day moving average, and less aggressive when below the average.

Worrying too much about taxes.

In general, I agree.  Taxes are a secondary concern, particularly for those who use stocks for charitable giving.  Donating appreciated stock is a home-run strategy for those with long-term capital gains.

Not paying attention to your investments.

This is true.  If you can’t evaluate you own investments, you should get a professional to do so.  By professional, I mean someone trained to understand how investing works, because few truly get how it works.  They should at least hold a CFA Charter, and hopefully show some competence beyond that to show that they have transcended the training of one with a CFA Charter.

My main points to you are these:

1. Don’t look through the rearview mirror.  Look through the windshield, and pick the stocks that offer the best returns now.
2. Only buy a new stock when its implied returns are better than most stocks in your portfolio.
3. Only sell a stock in order to fund a new stock with better implied returns.
4. Good investing is a lot of work.  If you can’t do it, get a professional to do it for you.
5. Consider taxes to the degree that it makes sense, and donate appreciated stock when you can.

The author’s six investing errors have a modest amount of merit, but the intelligent investor is dynamic, and adjusts to changing market conditions.  Your assets should be managed by those who are similarly dynamic, if you can’t do it yourself.