One of the most simple but costly mistakes many traders make is by rushing into a trade. Meaning, they made a trade where they didn't want, trading the wrong currency pair, bought instead of sold, etc...
Double check your trading plan, the order entry buttons, the currency pair you are trading, the time of day (so you are not off 30-60 minutes), basically it is a good practice to double check everything. Once you put a buy on when you wanted a sell, or you traded the GBP/USD when you wanted the USD/JPY, or when you wanted to close a position and you opened another, then you will see the importance of slowing down and checking things over again.
Another thing that can be very helpful is to not trade when you are tired, hungry, sick, or emotionally up set. So you could say, have a mental check up before you start to trade. By planning your trades you can gain more control over all the emotions, greed, fear etc.