How can I avoid marking up a toilet with a toilet auger?
I've had to use one a couple of times, and haven't had any problems with it marking the toilet.  The auger is flexible, so it's going to bend before you damage the porcelain... that is, unless you're using a lot more force than you should be.  IME, the clog is softer than the porcelain: the auger will take the path of least resistance through the clog, so you just need to apply enough pressure to keep the bit moving forward.

More instructions for using one are on about.com, ehow.com and acmehowto.com.