Buying on margin can greatly increase your gains or losses. Brokerages usually allow a bigger margin percentage for a day trading account but reduce the amount of margin available for positions held overnight. Normally a day trading account must have a minimum of $25,000 and can buy on margin at a rate of 4 to 1 giving you $100,000 in buying power, which is called day trader buying power. That number drops to 2 to 1 for positions held overnight, which can be called overnight margin buying power. That means that if you have 100% of your margin being used during the day, you must exit at least half of your positions before the close of the trading day.
Assess how much capital you're willing to risk on each trade. Many successful day traders risk less than 1% to 2% of their account per trade. If you have a $40,000 trading account and are willing to risk 0.5% of your capital on each trade, your maximum loss per trade is $200 (0.005 x $40,000). Set aside a surplus amount of funds you can trade with and you're prepared to lose. Remember, it may or may not happen.