When the price begins to drop is not the time to ever think of selling, except, of course you are very much in the profit margin. Then you must understand if the market prices will depreciate for long and hence sell quickly or else, if it is just a glitch then it is advised that you hold the fort!. The best thing to do when the prices fall again is to invest- yes buy more coins and hold them. It slows the downward slide (if you buy a chunk) and then gives you a potential profit in future.