We have taught nearly 1,000 people how to daytrade the mini-Dow or S&P emini index. Of those traders, nearly all were able to successfully trade our simple index strategy on their demo accounts (we only know of 2 exceptions that reportedly could not even get consistent on the demo. One man claimed that every single trade he made was a loss. In my mind that would be as hard to do as to make profit on every trade).
If your day trading strategy is consistently successful on your demo account, then what is the difference when you go live? Mindset! It all boils down to that in your trading (in my opinion this is true of life in general, but you see the results immediately in trading - especially day trading).
I really hate to call what we do as index traders, day trading. That is only because of the negative connotation the term brings to mind. Stock trading is what most people think of when they hear the term day trading. Regardless of what type of trader you are, you will have to come to terms with the fact that each trade depends on YOU. What frame of mind you are in at the time you place those trades will have a HUGE impact on how many of those trades are successful.
Most traders think that it all boils down to the technical and/or fundamental analysis of the markets. This is where they spend all their time and money, but they never get around to working on the mindset. They feel the real key is in becoming a great market analyst. However, the world is FULL of good market analyst (just watch CNBC or Bloomberg for examples) who are not able to trade. They too didn't have the right mindset and had to take jobs instead.
So what is the right mindset for a trader (or day trader)? That would take volumes of articles to answer. A good start is to read Mark Douglas' book "Trading In The Zone". Don't end your mindset training there, but it is a good start.
Another good exercise is to keep a traders diary. Write down what you were thinking and how you were feeling as you made your trade. Do this immediately after the trade so that you can be as accurate as possible. Do this on winning trades and on unsuccessful ones too. You should notice that on your winning trades everything felt easy and sure. Once you notice the difference, don't enter trades unless your mind is in the correct frame!
It's amazing how the human mind is able to pick up on the overall mood of the market. Douglas calls this being "In The Zone". We have always referred to it as getting a "Market Feel". Some traders have felt that it was impossible, while others gain that market feel advantage rather quickly. The difference is always in the mindset of the person. Some people are naturally much more in tune with their emotions, and they don't let them effect their mind while trading.
Many traders get hung up in all the technical tools that are available today. They reason that if they can just add the right tools, they will become successful traders. After working with hundreds of traders over the years, I can tell you for certain that you will NEVER be successful unless you have the right mindset.