Well, if it drops big without a corresponding news item, you should carefully redo your research and analysis. If you still think the stock was undervalued when you first bought it, you should probably buy more rather than sell.

These drops can be great buying opportunities. MIK, for example, has a tendency to overreact and drop 20% when they miss earnings projections by a couple pennies. Then it overreacts the other way when the next quarter beats projections.