As from Kathy Johnson of FeatheredGems.com:
Download a free trial music editor like Noteworthy Composer or Harmony Assistant, open the MIDI file of your choice (downloaded from a free MIDI file site), edit your tune, save the file as a MIDI 1 format file (NOT MIDI 0), and upload to the phone. You need to be able to read music to do this though.
If you use this tip, make sure you visit Kathy on her site and say thank you. 🙂
Of course, you will want to use my Cell Phone Upload tool to upload your ringtones to Sprint PCS, Alltel, AT&T Wireless, Cingular, TMobil and Verizon phones.