Clone Pokémon in Emerald

Here is how you clone a Pokémon and Items in Emerald the correct way.

Steps

Go to the Battle Tower in the Battle Frontier.
Log on to the PC to the right and deposit a Pokémon in a box.
Log off of the PC and save.
After save, withdraw the Pokémon.
Go to the lady closest to the PC. Select either level, it doesn't really matter. Then select 2 Pokémon you wish to compete.
After you choose 2 Pokémon, there should be 2 pauses. On the first pause, don't switch off the GBA/DS. After the pause, the lady would ask "Do you want to save?(again)." Now this is when you switch off the GBA/DS.
Switch on your GBA/DS and go to the PC. In the box where your deposit your Pokémon, should be your clone.
You can attempt to clone 5 Pokémon at a time, if you want. You can also clone items as well, by making the deposited Pokémon hold them.

Try this first with an unwanted Pokémon to see if you do it right.
This works because the save in the Battle Tower link section only saves the bag and party. It is possible to clone 6 Pokémon at once, but one other Pokémon will be lost forever.

If you switch off your GBA/DS on the first pause, you will corrupt your Emerald game and lose your Pokémon.