When fighting Bowser for the first time. Run around and collect the coins that he leaves behind until you have 1,000 coins. Notice now there is a 'M' next to your number of lifes. Now every time you die you gain a life. But every 1up you get takes a life away.

Enter the first bowser stage and get a thousand coins and a green m will
appear next to your live counter and when you die you get more energy.