When first looking into day trading you need to make sure you have the correct mindset and if you don't you are bound to lose money. The mindset is the most important thing about day trading. That is why you will hear many sites, books, and traders talking about Trading Psychology.
First and foremost, it's less stressful in the long run if you have the proper mindset. There is no going to bed with the fear of losing money. Once you setup the trade you can just walk away knowing you are following your rules and not making constant changes. This allows a cool head and calmer perspective. These are the traits that make a good day trader.
Make sure your methodology is sustainable and make sure you follow your strategy. This gets a lot of people in trouble as they look at the market and want to trade against a proven strategy. Don't!
Self-Control is a huge issue. A lot of people just do not have the restraint needed to trade successfully. Don't let your feelings control the trade. If you have researched it properly you will be fine in the long run. The best way to keep self-control is to set a goal on the number of losers you will withstand before you stop for the day or the number of winner before quitting (yes the odd are that if you have several winners in a row that a loser may arise. So stop while you're ahead. This will keep you from losing your valuable capital.
You need an iron will. As a trader you need to resist the tendency to have views and opinions of other traders influence what you are doing. What works for them may not work for you. Keeping your eye on the target will always help.