If you run out of lives hold down A and the press START.
After completing the game once the mushroom on the title screen will be replaced by Mario’s head indicating that a harder version of the game has been unlocked which features many tougher enemies.
Complete the game twice without turning off the power to make the level select appear. This is much easier to do on the 3DS Virtual Console version as you can save your progress.
On world 2-3, the level where Mario pilots a submarine you will find a plethora of coins spelling out Mario’s name and a 1-up heart in the wall afterwards. Grab as many coins as you can as well as the 1-up and then die when you reach the octopus enemy. You should be respawned just before the coins and can repeat the process as many times as you want.