Note:  To use this trick, you need to be atleast at the airship in World 8. 

When you start, you will see a few boxes and the spinner thing.  Break the boxes if you want, and then hop on the spinner and spin until the metal wall is all the way down.  Avoid the 2 enemies, break the next boxes if you want, then walk until you see an intersection of two 4-armed cannons and two normal cannons.  Now jump from one cannonball  to another for as long as you can before losing.  If you are good at it, you should get around 30 lives!  It is possible to get 99 lives doing this once.  Continue until you see 3 giant cannons, each on their own platforms.  This is harder, but as soon as the bottom one fires, jump on the giant cannonball, then the middle cannonball, then the top, then fall back to the bottom cannonball again.  Repeat this process without touching the ground.  If you are good at it, you might get 10 lives!  There are no more endless 1-up chances in this level, so either beat Bowser Jr. and the end, or lose one of your huge amount of new lives to exit the level. I will post more endless 1-up tricks for Mario games, so look out for them!