In addition to @InvestTrader, something else that dovetails into position size. Don't "day trade" your positions, and if you are, then ask the nice nurse for another shot and go back to your computer. Seriously, what I mean is, if I have a position open that is getting beat up, I try not to focus on the chart constantly, watching it's every move, this very often leads to me closing or altering the position, in other words, over trading. Sure, there are times I must focus on as specific trade/ticker, but I don't have the temperament to watch the screen, filled with angst like a day trader, finger hovering over the send button. And yet, I still often do it, so I have to constantly control that tendency. I think that one thing has cost me more money than any other.