This method patches the MeCCA.framework file enabling you to use your own custom ringtones and survives synching thru iTunes unlike other methods.

Step 1. Patch the MeCCA.framework

Download the patched MeCCA file here.

Place the extracted file into /System/Library/Frameworks/MeCCA.framework replacing the previous file.

Reboot your iPhone.

Step 2. Create your ringtones

Use iTunes to convert sound file to AAC (.m4a), then rename the converted file to:

ringtone_name.m4r

Notice: there must be a space before .m4r, so, a ringtone named ringtone.m4a should be renamed to ringtone .m4r.

Upload the renamed ringtone file to /Library/Ringtones, it appears in Settings->Sounds->Ringtones list immediately.