gamefaqs.com:

As you may know, jumping onto the flagpole at the stage’s end will net you some points (up to 8000). In this game, if you somehow manage to jump high enough and land on the very top of the flagpole, you will automatically receive a 1-Up.

ign.com:

This is easiet when you are blue shell Mario. In World 2-2, go past the three turtles, then past the cluster of power-ups, to find four coins enclosed by a cage of blocks.

When you take those out, you create a small, walled up gap where if you throw a shell into it, it will bounce back and forth forever. You would already have been attacked by a creature in a cloud throwing spiked shells at you. Stand close to it without stepping on the shell, and let the cloud creature keep tossing the spikeys into the gap, where the shell bouncing back and forth will take them out.

Press down (crouch) to hide in your blue shell so you don’t get hurt. Soon, every spikey that’s thrown into the gap and taken out by the bouncing shell will give you one more life. You’ll reach 99 lives in no time.

In principle this works wherever you can get a shell to bounce back and forth perpetually, and there’s an endless supply of enemies dropping in to be taken out.

ign.com:

In some worlds, there are cannons you can access (usually through taking secret exits). When you enter the cannon area, there will be a large cannon in the area, and a world number written below it. If you want to warp, simply drop into the cannon and off you go! Here is a list of the known warp zones thus far:

World 5 Warp #1: Take the secret exit from the World 1 Tower.

World 5 Warp #2: Take the secret exit from Stage 2-A.

World 6 Warp: Take the secret exit from the World 3 Ghost House.

World 7 Warp: Take the secret exit from the World 4 Ghost House.

World 8 Warp: Take the secret exit from the World 5 Ghost House.

ign.com:

First, each time you get to a boss stage, return to world 1. Play the first stage once through, getting all the power ups but the giant mushroom (you are forced to only be super mario at the end of the level if you gat it, without the fireballs). Instead, beat it and jump on the flag pole when the last two digits of the clock are either 66, 55, or 44. This way, a toad house with extra lives will appear. Repeat this until you have many, many lives to spare.

To cheese the bosses, play through one more time (without getting the big mushroom) and end when the last two digits of the clock say 99, 88, or 77. This will make a toad house with the giant mushroom powerup appear. It will replace whatever else you already have. Finally, go to the stage with your newly equipped giant mushroom, fireballs, and lives. Play through it, but don’t use your giant mushroom yet.

When you get to whatever boss you are playing, immediately activate the giant mushroom. Now you can defeat the boss just by touching him! I don’t know if the big mushroom will kill all bosses, but for most bosses, this works.

ign.com:

To get infinite 1-UPs, you needn’t go any further than World 4-5. There, head to the first pipe spawning Bobombs. Clear a way to the lower (indestructible) floor, but leave the left and right edges in tact. Grab a Koopa Shell and toss it into the pitt, and then position yourself far away enough from the pipe so the Bobombs continue to respawn. If done correctly, the Koopa shell will bounce left and right endlessly, racking you up 99 lives in no time.

about.com:

After you reach the mid-level point, you will come to an area with two rows of bricks. You must be big, but not mega-sized to perform this trick. Knock out the middle brick on the top row, then knock out the third brick in the bottom row. This will cause a vine to grow. Climb the vine and make your way to the end of the level above to reach the red flag secret exit.

about.com:

Just before you reach the giant Bullet Bill, you will see a pipe that is covered in blocks. Use the Bob-ombs to blow up the blocks and then access the pipe to reach the red flag secret exit.

about.com:

To locate the secret exit in World 7-4, you will need a mini-Mushroom (in storage, if possible). Make your way past the mid-level point and upwards to the left side of the screen (high up). You will eventually come to a flying “?” block. Use the mini-Mushroom to make Mario smaller and jump onto the block to stop it. Then, use the block to jump into the little opening in the brick wall. Make a “C” jump (backwards C) and jump up and to the left to land at the top of the wall. Go over and you will see a small pipe that only mini-Mario can fit in. Take this pipe to reach the red flag secret exit.

about.com:

If you beat a level with a time left of x11, x22, x33, x44 etc you get fireworks and the original Mario level completion song will play.

about.com:

Successfully complete a stage with the time having the last two digits of the same number to have a different color Toad House with bonus at the start of the next World:

• Have a time ending in 11, 22, or 33 for a red Toad House that awards items.
• Have a time ending in 44, 55, or 66 for a green Toad House with a mini-game for an extra life.
• Have a time ending in 77, 88, or 99 for an orange Toad House with a Mega Mushroom.