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.