this issues has been discussed a number of times. ckeck out modmyiphone.com for a method of making the plist sync proof. the method of creating a link (your understand when you read the posts) does not work for me. My solution works the best for my iphone, so i use it.
Please I cant find the post in modmyiphone, could you link the post for us?
Here is solution for mp3 ringtones. As we all know after sync iTunes will overwrite Ringtones.plist file with empty one, so all you need is just patch itunes.exe, find and patch out Ringtones.plist to Ringtones.smtng. Offset of 126.96.36.199 iTunes is: 0xB80ABE to 0xB80AC2 (5 bytes). You also can patch only one byte, so just get rid of .plist. Every time iTunes sync after this - it will look for another file and will leave your Ringtones.plist unchanged. All credits belong to me :-)
This way you can solve iTunes overwrite Ringtones.plist.