After you beat the game you can go to the Delfino Airport. When your there gather 100 coins to obtain a 1-up. Exit the stage and head over toward the cannon that sends you to Delfino Park. Dive in the water at the end of the street and at the sea floor is a hole with a free man. Get it and return to the Delfino Airstrip. it costs 10 coins which means you now have 90. Gather 10 more to get the 1-up and then leave. Get the one under the water again and keep repeating this trick for massive lives.