Things You'll Need
Instructions
Keep a saved copy of your Mii on the Wii remote. According to Technipages, you can do this by going through the following steps: Go into the Mii Channel from the Main Page. Select START. Select the icon at the top of the screen that looks like a Wii remote. Select the remote you would like to use, and if it asks you whether or not to format it, say YES. Follow the instructions; they will tell you how to select your Mii by holding A and B simultaneously and drag it to the save slot on the remote.
Go to your remote again once your Mii has been lost. Select the backup copy of your Mii, and place it in the Mii Plaza again.
Frequently update your Mii. The more often you do so, the less you will lose if your Mii gets deleted on the main system.