When you turn on the game and the words "Game Boy" show up on screen and you hear the chime, press buttons on the D-pad to change the color of the game.
Press the Select button on the pipe select screen to activate Easy Mode, which reduces the number of enemies and dangerous obstacles in levels for a less challenging experience.
When you collect all 5 coins and are ready to enter Wario's castle, try to get the bunny ears before you enter. It will make the stage easier. You can use them to fly over mace balls and fireballs, and can land on moving platforms easier. Also, you will have the advantage over Wario because you can fly above him and land on him to do damage to him.
You can take advantage of the fact that the game will instantly return you to your previous location in levels when you reset and reload the game to prevent yourself from ever using a continue. Simply pause the game when you are about to lose a life, then reset the game. Reload the file when you return to the Title screen and you'll be as good as new.