1: In multiplayer, press A to go into a bubble. This is very useful for Star coins. If a star coin is over lava, simply jump for it and press A before hitting the lava to obtain it unscathed. This can also be used in any bad situation to keep from losing a live. But make sure everyone dosen't go into a bubble at once or everybody will lose a life.
2: Many enemies can be defeated at once by 2 players ground pounding at the same time making a POW block effect. This is very useful for getting lives.
3: Here is how to get a mushroom house of any kind with only one player.If you have 2 remotes, put in a second character and go to any level. Press A on player 2 to make him go in a bubble so you dont need to play two characters at once.Complete the stage with the timer's last two numbers at 11, 22, 33, 44, 55, 66, 77, 88 or 99 to get a mushroom house, 11 or 22 gives a 1up house, 33-77 gives a item house and 88 or 99 makes a star house.
This is some tricks I use in the game, hope this was useful!