heres how i do it. and i luv it!

save the mp3 or acc or m4p song to your computer.
go to http://www.youmobile.org/mms.php
enter ur # and other settings and info
select ur file
hit send!

the file will b sent to ur phone in a NORMAL text message. it should come from a place kinda like mocheng@lg-vu.l.vu.com or sumthin.

hit options and tap "save sound as ringtone".
ur done!

GOOD LUCK!

NOTE: THIS SERVICE IS FREE. YOUMOBILE DOES NOT CHARGE FOR SERVICES. STANDARD TXT MSGNG RATES APPLY. NOT PREMIUM. NORMAL! ABSOLUTELY FREE IF U HAVE A TXTNG PLAN.