I think it's important to plan exactly how much you're willing to lose in a trade, and then follow through with that plan. It is equally important, however, to know what kind of stock you're dealing with, ie. the volatility of the stock. For some stocks, 5 or even 10% daily price swings are the norm. On the other hand, you have stocks like General Mills - GIS - that will typically move 1% or less on any given day. Therefore if you're trading a high volatility stock, you need to give it a little room to move in order to avoid getting forced out of the trade prematurely.
As an example, take a look at the chart for LJ International, Inc., symbol JADE. At the beginning of the month, this stock showed up on the StockCharts.com Bearish Engulfing Pattern Stock Screen. The stock was also falling from resistance at the 50 and 200-day MA's on higher volume. These were great reasons to short the stock.
Let's say you saw this bearish action and got short at the end of the bearish engulfing day at $7.56. Now you have to decide where to place your stop to protect yourself from losing too much money. A good place for a protective stop would have been above the highs of the entry day, around $8.43. But $8.43 represents about a 12% loss - which is quite painful. When trading a highly volatile stock like JADE - no pain, no gain. I crunched some numbers and found that over the last 10 months, on any given day, the difference between JADE's high and low was on average about 7%. If you hadn't taken this into account, you probably would have gotten scared and taken your loss. You would have missed out on monster profits.
Here's a daily chart, showing the sweet 47% profits that could have been made.
Here's a 30-minute chart showing the pain before the gain:











