Buy stocks on margin, shorting, etc. But companies are already levered, you don't need leverage yourself. You should not use debt on something volatile like stocks. If you do, do so lightly like Berkshire Hathaway. Say they invest 100$ in a company, they may borrow an extra 10$ to invest 110$ in said company. But 10/11 of the money is actual cash.
"My partner Charlie Munger (Trades, Portfolio) says there are only three ways a smart person can go broke: liquor, ladies, and leverage. Now the truth is - the first two he just added because they started with L - it's leverage."
"This table offers the strongest argument I can muster against ever using borrowed money to own stocks. There is simply no telling how far stocks can fall in a short period. Even if your borrowings are small and your positions aren't immediately threatened by the plunging market, your mind may well become rattled by scary headlines and breathless commentary. And an unsettled mind will not make good decisions."
Timeless Advice from Warren Buffett on Using Leverage
Why do you want to use leverage?