You can not "improve" existing files. Once compression has been applied, audio data is lost forever. That being said, Creative's X-Fi sound cards are quite impressive at being able to abstract the lost information and restore sound fidelity. External processors for mp3 players and the like are also available.
If you are talking about improving files that you rip in the future, you need to change the encoding settings. Ideally, you would use a lossless codec like .ape or flac.