Unlimited items
In the town of Cerebus, when Zakarie wants you to take your reward from the armory, enter, take the Mace of Disruption, Ring of Protection +3, Gauntlets of Ogre Strength, and anything else you may want, but do not leave. Instead, turn right, go forward, turn around again, go back to where you got the treasure, and you can get it again. Repeat this as many times as needed.
Item and character duplication
This trick is best done at the main menu or at a Training Hall.
Save the game. Remove one member of your party, so you have five remaining. Create a new character, for example "Bob", and add him in. Remove all of his equipment.
Transfer all the equipment you want duplicated to Bob. Remove him, and quit the game fully -- do not save.
Reload that game, and you will be back to your normal six party members, with the items you intended to copy. Remove that character again, and add in Bob again, and he will also have those items. Trade those items back to the party, and your party will have their items, and the duplicates that Bob had.
Remove Bob and add in your other members, save, and you will be back into the game, with two sets of items.
The character duplication trick is far easier, but only if he or she has not been in combat and can be modified. Rename your character using the "Modify Character" command, then remove him or her. Quit without saving, and reload. Your renamed party member will be in the pool of characters, and your original character will still be in the party. This is also a nice way to gather up useful items from new characters in Dark Queen of Krynn, where some of them receive potent magical items not found in Death Knights of Krynn.
At either the main menu, or a Training Hall, trade all the items you want duplicated to one character. Remove that character from the party. Add them back to the party. Remove them from the party, but notice that the game asks if you want to overwrite the original. Answer "No". Give the character a new name (for example, "Mule_full"). Add "Mule_full" to your party. Take all the items you want from them. Remove "Mule_full" from your party. When prompted to overwrite, answer "No". Give the character a new name (for example, "Mule_empty"). Add "Mule_full" to your party and repeat. On subsequent times, you can overwrite "Mule_empty".