You will need the following:

-Toy Box Chapter Completed.
-First Attack Badge equipped.
-Become a high enough level to kill shy guys with a First Attack Badge.
-To open the pipe that leads straight to Bow's Mansion.
-At least 18 coins.
-Make sure Tayce T. can cook 2 items at once!

Go into the Toy Box. As soon as you enter, go west. Avoid the Yellow Shy Guy and the Dancing Shy Guy on your way. When you get into the room with Shy Guys that hold items, mug the following items from them:

-2 Cake Mixes
-A Mushroom
-Maple Syrup

Leave the Toy Box and head for Bow's Mansion. Go into the Boo's shop and buy a Super Shroom. There, they only cost 13 coins, instead of the high price of 20 coins in toad town. Go back to Toad Town and buy a Mushroom for 5 coins. Head over to Tayce T. and cook the Cake mixes with the Mushrooms. You'll get 2 Shroom Cakes. Cook the Super Shroom with the Maple Syrup and you'll get a Maple Super. Go to the Store to your left and sell all three of the items you made. You'll get a total of 57 coins when you do this! Repeat these steps as many times as you want. It's a little Time consuming, but it's worth it!

Mushroom + Cake Mix = Shroom Cake (You make 20 coins when you sell this.)
Mushroom + Cake Mix = Shroom Cake (You make 20 coins when you sell this.)
Maple Syrup + Super Shroom = Maple Super (You make 35 coins when you sell this.)

Total so far: 75 Coins - Mushroom (5 Coins) = 70 Coins - Super Shroom (13 Coins) = 57 Coins

Grand Total: 57 Coins - x AS MANY TIMES AS YOU WANT! - 18 Coins for each time.

Example:

75 Coins x 5 = 375 Coins
18 Coins x 5 = 90 Coins
375 Coins - 90 Coins = 285 Coins

In other words, This secret CAN and WILL get you as many coins as your heart desires! (Yes, even 999 Coins.) RATE!!!