Original Mario Bros
In world 3-1, go to the end of the stage where the stair case to the flagpoll is. Let the first turtle walk down all the way, but stay on the staircase. Next, stand as close to the block as possible, and when the next turtle walks on the block your facing, jump lightly and you should instantly continuously bounce on the turtle giving you mega points and eventually give you unlimited lives. You can get up to 127 lives this way.

Lost Levels
On level 1-1, hit the second block to the left to get the first mushroom of the game to come out, being careful not to hit the turtle walking there. Next, get the mushroom, power up and go back underneath the turtle and the blocks. Time this jump and break the block so the turtle is trapped on the right, walking back and forth, left and right from the edge of the block it's walking on and the block it can't walk through. Then, break the block left of the used powerup block, jump up, break the blocks above you all except for the one above the turtle walking back and forth. Jump against that, so that you land on the turtle and you'll bouncing continuously giving you again, mega points and eventually 1-ups until you get 127 lives.