From “Reminiscences of a Stock Operator” by Edwin Lefevre, the 1923 classic pseudo-autobiography of legendary trader Jesse Livermore:

… I didn’t always win. My plan of trading was sound enough and won oftener than it lost. If I had stuck to it I’d have been right perhaps as often as seven out of ten times. In fact, I always made money when I was sure I was right before I began. What beat me was not having brains enough to stick to my own game — that is, to play the market only when I was satisfied that precedents favored my play. There is a time for all things, but I didn’t know it. And that is precisely what beats so many men in Wall Street who are very far from being in the main sucker class. There is the plain fool, who does the wrong thing at all times everywhere, but there is the Wall Street fool, who thinks he must trade all the time. No man can always have adequate reasons for buying or selling stocks daily — or sufficient knowledge to make his play an intelligent play.

Sometimes the best play is to not play at all. When playing the market, you have to let the opportunities come to you, and take advantage of them when the odds are in your favor. If you don’t, you’ll get very frustrated — and you’ll lose money.