compressing music files

hello, i have an mp3 player, curtis and i have mp3 files on my computer i was wondering if there was any way that i could compress those mp3 files to make them smaller so i could fit more of them onto my mp3 player. does anybody know a way? thank you in advance.
Who is Curtis?

Lemme take a wild stab and guess the bulk of your files are goth metal?

Sure, you can compress an mp3 to ever smaller files. The trade off is audio quality, which is going to get worse and worse. The basic gist is that by going to a smaller file size, you get a lower effective sampling rate, among other things, and the player has to interpolate more--hence worse sound. But, if you want your songs to sound like they are being played through a telephone, get yourself a copy of LAME and process your files changing the settings to lower and lower audio quality.
Maybe we could start another cataGORY for people who want LOW-fi. When you start with MP3 which is really poor quality, why would you want more of it? I'm not sure why some people get into this?!?
actually no its mostly techno and trance but yeah i tried that and regarding the audio quality uh i guess i can live with the fewer songs on there cause some of them just sounded horriable.
If you try to further compress something that's already compressed, you will indeed get something that sounds "horriable." That's because the stuff that codecs strip out is already gone. If you go back to the original uncompressed file (if you have it), and re-compress it at a lower bit rate, it might still be acceptable to you. Sound quality vs. file size is a trade-off. Every listener has to decide for himself.