You can't ever get it back to the original quality, but depending on the encoding rate and type of music you listen to/what you're listening on, it may not matter. While mp3 certainly isn't perfect, it does a fairly decent job for what it's supposed to do, especially if you use a quality encoder like LAME.
If you've got a decent pair of headphones download FOOBAR 2000 with the abx extension and try abx'ing some of your mp3s with wavs of the same song. I think you'll be surprised at how difficult it is to tell the difference.