The common mistake trader's make is THEY FIGHT THE TREND. Certainty doesn't exist in trading. It's about trading high probability setups. Trading the path of least resistance will help to stack the odds in your favor. Avoid trying to call tops and bottoms. It's a suckers game. Don't try and buy the beaten up stocks. They are beaten up for a reason. Selling the strong stocks is fighting against the grain as well. 

Keep it simple. Go long in stocks that are moving higher and go short stocks that are moving lower. Be patient, wait for pullbacks, consolidations, or fresh breakouts but make sure you are staying on the right side of the market. 

Fighting the trend is like standing up to a tidal wave head on and expecting the wave to break an inch before it runs you over.

The first thing you should ask yourself when looking for trading opportunities is WHAT IS THE CURRENT TREND? There can only be 3 answers. UPTREND, DOWNTREND or CHOPPY.