just about all of your significant losses come from news being released that affects the stock after you have placed your trade, to mitigate this risk make sure the stock doesn't have upcoming earnings annoucements, stock sensitive pending announcements or any clouding market rumours, sometimes the reason is unknown, you do have to do some homework, it takes me no longer than 5mins to search for stock news for multiple sources, this is where a full service stockbroker can help, he is your man on the street and can let you know what the market talk is on any given stock.
To reduce your losses try employing a 15 day time stop and close your position within this timeframe regardless of PnL, also you can try sticking to stocks that have a history of very few stock price sensitive news items, BHP would be the opposite of this example and I don't trade it, too many variables at play, ADRs, governments, contracts, etc.....very hard stock to trade. If you position size correctly you can takes losses no problem, you have it to be able stomach the losses and not take it personally or give reason to change your method, this is why succesfull trading flies in the face of how we are brought up to function, if we do something and its wrong that we need to change how we did it, whereas in trading this doesn't apply, you make more money from sitting on your hands than ''doing something''.
Its a numbers game, treat it just like that, numbers, don't measure your losses in the material equivalent like I just lost $500, that a nice weekend holiday gone, try and trade with the attitude, ''right im going to make 100 trades over the next 3months and I don't care if I breakeven, it will be a good experience and I will learn alot'' going in with this attitude is good because your not expecting fireworks and any profit is going to please you.
Remember this is speculating, making an educated bet, not science. Trading is not placing a trade then thinking there might be a chance of a loss but it always turns into a winner for some sort of cheap thrill, you have to take losses, learn to love them, and treat like one step closer to another winning trade.