Three basic rules I have been following lately: 1. Always take profits, (50% profit on a seemingly struggling level II is better then a 10 percent loss 3 hours later). 2. Sell when the stock is rocking, if it is not rocking, continue to step three. 3. Sell if a stock falls below a prior support/resistance point. That being said I don't really look at percent, but resistance/support. Have I been wrong and sold? You bet! But I have been right and kept myself from loosing more money more times then not. Remember there is always another play right around the corner in which you can make up a smaller loss, but a larger loss is hard to make up quickly. These may not work for everyone but they work for me cause I am small potatoes.
Epic and Tec have some GREAT rules for trading on their website, if you have not already been there go take a look....and then read them every day for like a month. It will sink in :).