My rule of thumb on this is simple. If I didn't have it and I had a chance to buy it at the price I would get by selling it, would I buy it? If the answer is yes then keep it. If it is no then sell it.

You could also do the coin flip trick. Heads-sell, tails-keep. When the coin is in the air you will know whether to sell it or keep it. You won't need to see it land....