This trick will allow you to get 55 of any item in the game. It does not work for weapons, spellbooks, or armor. It can only be done for items that any unit can carry. Select any unit, and fill that unit's items all the way except for the last two slots. For example:

Heal leaf
Heal leaf
Heal leaf
Heal leaf
Heal leaf
Heal leaf
Empty
Empty

Next, place the item you wish to duplicate in the very last slot. For example:

Heal leaf
Heal leaf
Heal leaf
Heal leaf
Heal leaf
Heal leaf
Empty
Heal Pack

Next, remove one character from a unit that can carry only one item. You can check how many items a character can carry by looking on the info bar above the character when you place the pointer over the top of the character. Now the items in that unit should look like this:

Heal leaf
Heal leaf
Heal leaf
Heal leaf
Heal leaf
Heal leaf
Heal Pack

Next, go to the "Carry item" command, and remove that item which you want duplicated from the unit. When this is done, the item will look like this:

Heal Pack 55/01

However the last number, after 55, will vary depending on how many you had before. Now, go to the "Item's command", and you will see the item you want to duplicate. It will look like this:

Heal Pack 55/01

You must sell two more than the second number. For example:
If you had 55/01, sell three
If you had 55/02, sell four
If you had 55/03, sell five
etc.
The item should look like this:

Heal Pack 55/54

Now you can use 53 of that item, but make sure you save the last one so you can repeat the trick. Note: If you exit the organization screen, the first number will disappear, so it will look like this: Heal Pack 00/54.