iTunes is good, select Preferences > General > Importing settings (or something) and select format you want. Then select songs you want to convert in your iTunes library and right click and select "Create --- version"
Are you saying you already have the files and you want to compress them further through zipping them or something like that?
Compressing mp3s further using ZIP or RAR or whatever results in negligible file size reduction due to the nature of an mp3 file. It's already compressed just about as much as possible.