This is for those of you who are having trouble syncing ringtones to your iPhone. You may have created or purchased ringtones, and believe they are in your iTunes library but after syncing they still don't appear on your phone. There are a number of possible reasons for this. Here is a checklist to follow when syncing ringtones to your phone:
Confirm that they really are ringtones: right-click on the files in iTunes, select "Get Info", then verify on the Summary tab they say "Kind: Ringtone". If they don't then you must first convert them to ringtones and import them into your iTunes library before syncing them to your phone. (Guides for how to do this are shown in the References section at the bottom.)
If they are purchased ringtones, confirm that your computer is authorized for the Apple ID(s) used to purchase them (by going to iTunes > Store > Authorize this computer).
Verify that the the ringtones are 40 seconds or less.
Confirm that you have checked "Sync only checked songs and videos" on the Summary tab of your iTunes sync settings.
Confirm that you have checked the box next each of the ringtones that you want to sync to your phone in the Tones folder of your iTunes library. (If you don't see the Tones folder check the Reference section below for how to fix this.)
Confirm that you have checked "Sync Tones" on the Tones tab of your iTunes sync settings. (If you don't see the Tones tab then you haven't yet imported any ringtones into your iTunes library.)
When everything looks correct sync your phone. Custom ringtones will be found at the top of the list in Settings > Sounds > Ringtone.