Here is another way to easily get 1000+ lives.

Go to world 1-4, at the beginning go up the stairs that the two koopas walk down. At the top, there is one koopa walking back and forward in front of a small pond. When he is turning around on the left wall jump on his head, don't jump on him the second time just yet! When he is in his shell, kick him toward the small platform in the middle left of the pond (should be right in front of the shell you just hit). After this, he should be bouncing back and forward in between the two platforms. Wait for the goombas to get killed by him (They should walk toward you) and then jump on top of the shell! STAY STILL AFTER JUMPING ON IT! Your lives will now shoot through the roof! Do this until the time runs out and do it again until you have the max amount of lives!

After you get the Statue Leaf try becoming Statue Mario during a boss battle. Mario will take no damage while in this form making the bosses crazy easy to beat. Their attacks become useless against you.

In World 1-2 it is possible to collect Unlimited Lives! Right after the Checkpoint Flag, in the area with the 3 Koopa Troopa's, collect the Super Mushroom/Fire Flower in the lone Item Box in between the Triple Item Boxes, as you will need to be Super Mario to pull this off correctly. After doing so, align Mario with the end of the last Triple Item Box, face it, and wait for the Koopa Troopa to walk towards you. When he has almost reached you, jump up high enough so you will jump on him, and do absolutely nothing.

If done correctly, the shell will hit the wall, and just as you are about to land, it will arrive at your feet, bouncing you back up, and you'll kick it again, continuously earning 1-ups until the time runs out, or you move.

In World 1-2, just as in the first Super Mario Bros., there is actually a Warp Zone!
To reach the Warp Pipe, normally traverse the level as you would, but before exiting through the pipe at the end, stand at the bottom the small ramp that leads to the Ink Piranha's, and you can see an opening in the ceiling! Wall jump, and you will be walking on the ceiling. Walk left to snag a 1-Up, and walk right to find an Orange Warp Pipe to Warp you to World 2!

Normally once you've completed a mystery box it becomes dormant and unavailable until 24 hours later. There is a way around this though.

On the 3DS system settings set it to 1 day later than it currently is. Save settings and then go play this game. All Mystery Boxes will be available again.

On world 1-1, if you go down as far as possible then right as far as possible, you will notice a cave blocked by a crate, if you smash the crate then go through the cave you will get a 1-up mushroom then exit the cave go up to the higher part and jump down and right onto the coloured blocks then run down and scare away the bird and another one will appear.

Warning: this Easter Egg is uber creepy. Viewer discretion is advised.
At the end of level 4-4 if you walk past the flagpole and stand in the upper corner facing the fence for a while you will see an eerie figure appear in the background.

If you see a field full of dandelions (try Level 1-1), blow into the microphone! Pretty!

Figured this out when I sneezed. <img src="http://cdn.staticneo.com/neoassets/smileys/smile.gif" border=0 vspace=2 alt="" />

(SPOILER)
In the Final Battle at the end of World 8 with Bowser, one of his attack methods is to throw barrels at you, just as Donkey Kong does in the original Donkey Kong™ arcade game!

If you wait around long enough at the starting screen, it will change to a mini 3D area. There are a few Toads and a structure in the middle. It appears to be for learning or practicing jumping, attacks, and using the 3D feature.

While playing the first level, you will see two trees standing side by side after the mid way flag. Climb up the one to the left and a hidden Super Leaf will float out.

When you lose a power up when you're hit by an enemy you will start to flash, and if you press start at the right time, Mario/Luigi will be invisibe.

In World 5-2, the screen will be in a top view similar to LoZ. If you light the torches in the room to the right of the round room, you will also hear an LoZ medley play.

After you beat the game once, you receive a Star on your file in the file select screen. If you haven't died more than 5 times in a row on any level up to that point, the Star will sparkle.

[Spoiler]
Remember Bianco Hills in Super Mario Sunshine, and how the second Shine Sprite had you battling Petey Piranha inside the windmill? Well an opening similar to this battle takes place in the Final Battle against Bowser.

When you first reach Bowser at the top of the castle, through the red doors, you'll run into the middle of the room, and Bowser will punch the ground in front of you. After this, he will roar, and the floor will crack, where Mario and Bowser's eyes will snap to the floor, and slowly rise to meet each others face-to-face, and the floor will crumble, where you will both free fall until landing at the site of battle, just like you did on the top of the windmill with Petey Piranha in Super Mario Sunshine, albeit faster.

Remember the Mario Drawing Song that premiered on Flipnote Studios? Well that very song can be found in Special World 8, as the Level Select Background Music!