Gold Diamonds:
There are two 20,000,000G Gold Diamonds in this game. One you get after you catch your 50th boo (King Boo doesn't count as a boo). Another way is a long process: After you get the water element medal, go outside through the left door and beat the dog. When you finish, get some water and spray some on the green dot in the dirt bed to the right of the doghouse. It should grow a little. Then beat Bogmire. After you finish, give the plant a second dose, and it should turn into a flower. Finally, after you beat Boolosous, give the plant a third and final dose. After you do that, you should get alot of money and a Gold Diamond the size of Luigi's head!

Get Lots of Money:
After defeating the Butler in the Butler's room, use the Game Boy Horror and scan the mousehole in the corner by the door. This will open up a portal to a secret room where, after defeating the ghosts there, you can get a ton of money.

Try to vacuum up any gold ghost mouse you see, you will get money.

Use the Game Boy Horror to scan any pieces of cheese you see in the mansion. This will automatically attract a gold ghost mouse. Vacuum it up and receive money.

After defeating Boolossus, water all the plants on the balcony. Many of them will give you jewels and gold bars.

After defeating the dog in the yard, water the budding plant in the garden on the right by the fountain. Remember to visit this area periodically through the game and water the plant each time. Every time you visit and water it, the plant will grow bigger. Eventually it will turn into a flower. The next time you visit it, it will look like a round pod and will dispense a HUGE diamond.

In-game reset:
Hold B + X and press Start while playing.

Unlock Another Mansion and gallery mode:
Finish the game and save.

Shortcut to the main/lobby of the mansion:
Switch to your Game Boy Horror's first person mode and examine any mirror in the mansion with A. After a really trippy warp sequence, you will find yourself in the entrance hall of Luigi's Mansion

Heart:
If you run low on power, run to the first level bosses room. (after beating him of course) and there will always be a 50 pt heart in the chest next to the door. If your under 50 hit points left. Exit room and go to main foyer and return to room and repeat. Sometimes it's out of the way...but it will keep you from dying.