Be Humble in Bull Markets
by Andrew Beattie
Sometimes, particularly during a bull market, gains are not dictated by investor actions as much as by having money in the market, so don't allow yourself to become overconfident. Overconfidence often leads to overtrading, taking unnecessary risk and eventual losses when the bull turns bear. Also remember that you incur commissions every time you trade - this expense can often erase profits or increase losses.
Source: http://www.investopedia.com/articles/basics/07/10commandments.asp