Ok i have used this technique to get a lvl 100 typhlosion before whitney.
1) Make sure the Pokemon you want to clone is holding any item you want to also clone. IT DOESNT NEED TO BE IN THE FIRST SLOT!
2) Make sure you are on ANY empty box.
3) Save the game.(it helps if its right in front of the PC cause then you can access it quickly)
4) Deposit the Pokemon you want to clone IN THE EMPTY BOX.
5) Change the box to ANOTHER EMPTY BOX. (I'm not sure if it needs to be empty, but it worked for me.
6) You can turn the power off AFTER the "S" in SAVING appears(not before or the game will not have started overwriting the file) but BEFORE the screen changes again. I'd just turn it off when "DON'T" appears.
7) Turn back on and you should have 2 of that Pokemon: one in your party, and one in the box. Both will have the item if you gave them it.
REMEMBER to follow EACH step as it is written or else you might lose the Pokemon, or, on a lesser degree, the game will simply save and the pokemon will be in an empty box.
Hope I helped!
Diamond 2191 7706 6159 < to trade shinies
Pearl 3265 7613 9860 < to battle