After you collect all 120 stars go outside the castle. Where there was a closed grate the whole game there is now an open cannon. Shoot to the top of the castle where Yoshi, our favorite pal will be waiting. Talk to him to get a new jump and 100 lives.
Talk to Yoshi and now you'll have a lightly higher, star spewing triple jump!!
This is a very long and frustrating trick but it may be worth it to some people. in a bowser level collect 1000 coins. Collect coins from Bowser's fire breath. After you collect 1000 coins there will be an M in front of your lives. Now, every time you gain a life you'll lose a life and everytime you lose a life you'll gain one.