For long-term investors in quality businesses, deciding to sell down or fully exit an investment can be an agonizing process. Behavioral biases and personalities can complicate these decisions.
I use rule #2, which I call the economic sell rule. In trading individual bonds, a good bond manager has a handful of rules to figure out whether swapping one bond for another is a good idea or not. Things are more vague for a manager of stocks, but most managers should have a rough sense of how promising each stock is... which could be expressed by position size. Thus virtually all of my portfolio trades are swaps of one position for another, which forces me to be businesslike in my limited trading.