Easy Leveling
Locate a spot with a star in a box next to a save point. Save your game, and then use the star to defeat as many enemies as you can. After the star wears off, enter another battle and let the enemy kill you. When you begin your game again, you'll be back at the save point and still have all the experience you gained. The star will be back in the box, and you can continue leveling with this technique.
Infinite Coins
On the path leading to the town of Rosa, you'll see an area featuring a group of Shy Guys standing on top of the treasure boxes. Go toward the exit at the bottom-right side of the screen, and you'll find a box next to the door. Jump up and hit the box a few times to collect coins, but don't empty it. Exit through the door and return. The coins will be replenished.
Geno Whirl Trick
After Geno joins your party, you can use his Geno Whirl ability to score some major attack points during battle. Geno Whirl will cause a tornado-like wind to come across the screen. Press the "Y" button just before the wind leaves the screen, and it will result in 9999 damage. This technique only works against normal enemies, not bosses.
Free Bracers and Energizers
When you go to Yoshi Island and ride Yoshi, speak to the pink Yoshi by the goal line to get three Yoshi Cookies. Take these items to the volcano and do battle with the Oerlikon and Magmus enemies. Use the Yoshi Cookies to transform these enemies into bracers and energizers. When you run out of Yoshi Cookies, you can return to the pink Yoshi and acquire more.
Secret Red Essence Locations
The Red Essence is a handy item that makes a character invincible for three turns during battle. You can find one by going just beyond the first save block in the Forest Maze area. Jump around on the right side of the walkway until you locate the hidden box that contains the Red Essence. If you have the Dream Pillow, you can acquire three Red Essences by sleeping at the Nimbus Land Inn. Toad will be there when you wake up, and he will give you the items.
Infinite Frog Coins
In Land's End, go to the sandy area where the armored ant pops out of the ground. You will get a frog coin every third time you jump on his head. You can continue jumping to get as many frog coins as you wish.