If you are already invested in a particular stock, I like JoeTaxpayer's answer. Think about it as if you are re-buying the stocks you own every day you decide to keep them and don't set emotional anchor points about what you paid for them or what they might be worth tomorrow. These lead to two major logical fallacies that investor's commonly fall prey to, Loss Aversion and Sunk Cost, both of which can be bad for your portfolio in the long run.
To avert these natural tendencies, I suggest having a game plan before you purchase a stock based on on your investment goals for that stock. For example a combination of one or more of the following:
I'm investing for the long term and I expect this stock to appreciate and will hold it until (specific event/time) at which point I will (sell it all/sell it gradually over a fixed time period) right around the time I need the money.
I'm going to bail on this stock if it falls more than X % from my purchase price.
I'm going to cash out (all/half/some) of this investment if it gains
more than x % from my purchase price to lock in my returns.
The important thing is to arrive at a strategy before you are invested and are likely to be more emotional than rational.
Otherwise, it can be very hard to sell a "hot" stock that has suddenly jumped in price 25% because "it has momentum" (gambler's fallacy). Conversely it can be hard to sell a stock when it drops by 25% because "I know it will bounce back eventually" (Sunk Cost/Loss Aversion Fallacy).
Also, remember that there is opportunity cost from sticking with a losing investment because your brain is saying "I really haven't lost money until I give up and sell it." When logically you should be thinking, "If I move my money to a more promising investment I could get a better return than I am likely to on what I'm holding."