All new Sprint phones will play standard MIDI files. Save them on your computer and upload them to the web using a site like www.pcspix.com. Once the files are uploaded to your PCSPix account, you can access them from the web using your phone.
I'm such a dork about my cell phone. I have specific ringers for each person. If someone is showing up that's not in my book, it plays No Surprises by Radiohead. Other ringers I use are Rapture, Atomic, Our House, Da Funk, Jenny 867-5309, Just Like Heaven, and Talk Show Host. I'm probably the one annyoing Nag in Pizza Hut or during an exam.
The time you wear them also depends on what you do. If you do what the doctor asks (rubber bands, headgear at night, retainer afterwords) then you'll stay right on track. If you play sports at all, get a mouthpiece.