Audacity. Its free. You open up the MP3 file in Audacity, highlight the section you want to use as the ringtone, and you cut it, paste it in as a new file and export as MP3. Sounds complicated but it doesn't take long at all and when you're exporting, you can choose the bitrate of the MP3 along with the other features that LAME runs on.
http://audacity.sf.net
Oh and btw, myxertones doesn't use MMS. If you browse their site directly, you can send the ringtones right to your phone. It sends through a text message, which you open up the link in your browser and download the MP3 directly.