How to avoid dying:
When you are going to die (except falling) pause your game and choose EXIT AREA. You will be teleported to the beginning of that particular level.

Sirena Beach:
When you have Yoshi, go to the roof above the fountain where you started. Have yoshi eat the pineapple in the warp pipe on the roof. Go inside to get to Sirena Beach.

Mario game clips:
During the intermission sequence where FLUDD (the water backpack) identifies Mario, you can view short clips of past Mario games, including Super Mario 64, in the lower left corner.
User Cheat by James777

Select files with moves:
When you are selecting a file you can do a Spin Move and the Side Jump to get onto the blocks.
User Cheat by James777

Sand Bird and red coins:
One of the red coins is on a tower not on the bird but be careful when you get off his wing.
User Cheat by pingpong71392

How to dive:
Go get the rocket nozzle(red box)and rocket up to the top of the shine gate. now jump off in to the water while holding in B (red button)
User Cheat by twinnick

Get the ending bonus:
Finish and save the game. You can pay 10 coins to use a boat near the clock tower where Yoshi is found to return to the Airport level at the start of the game.

Get a shirt covered with Shine pictures from the man wearing sunglasses gives Mario sunglasses normally the game.

Secret Manhole
There is a secret manhole on Plaza Delfino's Beach, a few steps from the 2nd Umbrella.

There is a LifeMushroom inside(along with 8 coins).  
User Cheat by fillygrl

Blue Coin:
If you use the underground tunnel you can go through a house with a blue coin.
User Cheat by sdsdfsdfsd

Pionta Village Coins:
Ya, it\'s pretty hard because it seems like you can only get 30 coins in Pionta Village. In addition to breaking boxes, spray the fires on the mushrooms untill you see smoke, then ground pound (A+R)in the center of the mushroom and coins will appear. Also ground pound the big nose on the statue that has water coming out to feed into the water tub. It will not only give you coins, but will also give you a blue coin!
User Cheat by djbfdbfhbbb

Different ending screens:
Finish the game with 119 Shine Sprites or less you will get an ending screen in which el Piantissimo inspects the paintbrush.

Finish it with all 120 Shine Sprites, the ending screen will show all the characters in the game.

There are 240 Blue Coins and 120 Shine Sprites. You need 50 Shine Sprites and have level 7 completed on all seven stages to get to level 8. You only need 117 Shine Sprites to complete the game to get one ending. Get the last three Shine Sprites to see a different ending. Get the shirt and glasses for a different appearing ending. 

Pianta and Sunglasses locations:
Below are the Pianta man that gives you the sunglasses is located in each level.
 

Delfino Plaza: Fruits Street.
Bianco Hilles: Near the start point.
Ricco Harbor: Near the start point.
Gelato Beach: Beach house.
Pinna Park: Near the start point.
Sirena Beach and Noki Bay: Not found in this level.
Pianta Village: In the village during the daytime only.

Where are the secret levels:
After almost every obstacle course, there is secret level in which you go to the obstacle course and Ground Pound a red button to find eight Red Coins in a certain amount of time.

Get easy lives:
Whenever you complete a level, go to the water directly next to where the boats go in and out. Dive all the way to the bottom and you will see a 1-Up Mushroom. Whenever you return from a level or start from the beginning, it will be there.

Get extra stars on secret levels:
After you complete a secret level, go back to that episode and replay the secret level. This time there will be a red hump on the ground. Ground Pound it and get the red coins. There will be a time limit, but you have FLUDD to help you.

Clean easily:

Instead of taking a bath each time Mario gets dirty, do one or two spin jumps.

Jump onto buildings:
Use a Ground Pound on a manhole cover then immediately jump up. This will send you high into the air. Use a Hover Nozzle to get on the building. REMEMBER: Some items, such as Shine Sprites and Blue Coins, can be found on roofs.

Jump higher:
Run in one direction, then turn in the other direction quickly enough so that Mario stirs some dirt up. Then, jump immediately in the air. Mario will jump higher and do some flips.
 

Spinning jump:
After jumping, you can do a spin by rotating the Analog-stick once in a circle.
 

Sprinkler Water Spray:
To execute the Sprinkler Water Spray easily, do a Spin Jump and hold R.
 

Yoshi's Spritzer Squirt:
Press R, then press A.
 

Cover more area with your FLUDD:
Jump three times to do a forward somersault. As you roll, shoot your Nozzle Cannon and spray a long distance in front, behind, and below you.
 

Avoid losing health when falling:
When falling from high places, press L to do a Ground Pound. This will make you fall very fast and not lose health.
 

How to swim faster:
When swimming, switch to your hover and press R. It will give you a little boost while swimming

Spray birds:
There are birds in almost every level in the game. Spray a green bird for a Yellow Coin, a blue bird for a Blue Coin, and a yellow bird for a Shine Sprite.