What are good rule-of-thumb guidelines for selling stocks?

The golden rule above, needless to say always works. For anyone who buys and sells stocks, you need to have a strategy and commit to that strategy.

For example I know a lot of day traders, who have a day-trade strategy of -3% (sell), and +1.5% (sell). So if a stock they buy at US$ 100, hits US$ 101.50, they sell. If the stock goes down to US$ 97.00 they sell. They do not enter into the market till the next trading day. It is self-discipline that they exercise, because at the end of a losing day, they still have 97% of wealth.

Some buy stocks for the long run, if that is the case, then buy and forget - till such time you have allowed yourself to sell it off. For example, some people will buy a stock and would like to own it for a minimum of 5 years, and they do just that. They simply do not bring that stock into trade for 5 years. All this is backed by reasoning in form of dividends, research, your outlook, etc. So YMMV.

A very important point when considering selling stocks is to ask, what will you do with the money next? Enter the market again? Is the market too high? Is a correction due? etc.