Lets say you want to pick a certain part out of a 5 minute song to be an ring tone on your phone, you just split it into a 30 second segment or however long you want it to be so its not the whole thing and it is the only part you want to hear
I use Super MP3 Editor to, well, edit my MP3's. Google it, I'm using my PSP to post this, so I can't remember the link. I think it is a trial, but you can still use it after the trial time. It has all the same features after the trial.