Money Management is the key to successful stock day trading.
If you don't know this .. don't trade.
You need to understand Risk in terms of "R" multiples.. and risk a certain percentage e.g 1% of your total funds to trade. As you make money, you risk more.
I'm researching into increasing the risk fixed percentage model into a dynamic one - that is, when you're making money, risk even more. So effectively, your money will grow even faster.