The Pumie, pumice stick, is probably the easiest and most successful way to get rid of the rings that I ever found. Usually the rings are hard water mineral deposits at or near the waterline (or below waterline) in the toilet bowl. The mineral surface is a bit rough, so stains stick to it...even iron from the water for rust stains. They are often a couple of dollars for a bar/stick at hardware, home stores and some large grocery stores. They last a long time too, just keep them wet and dipping in the water when using...sounds like they are scratching, but are softer than the porcelain...but harder than the mineral deposit.
I used to have the toilet ring problem all the time because we have very hard water. The pumice stone always worked best for me. I never found a liquid that would fully remove a real toilet ring - not even industrial strength acids.
After you get the ring off, make sure you coat the toilet bowl with that new self-cleaning coating. I did and I have been ring free for half a year and I don't barely have to clean it at all. You can find it at SelfCleen.com