I found a few great sites on eraser carving today. I had practiced on erasers when I took printmaking, but I was just playing with line. Some carvers, though, have created some really impressive stamps.
If you are interested, the Carving Consortium has a really nice gallery of images, though browsing isn't very user friendly.
If you just want to play around with it, Stamp Mania Australia has clear, straightforward instructions to get you started. If you are serious, though, or a perfectionist like me, check out this page by Carolyn Hasenfratz of Lime Green Evolution. It's much more in-depth, and has great diagrams and examples. There's also good advice on choosing images to carve.
Another good site is Tabloid Trash. It's just an archive, now, with cross-links to the Carving Consortium, but there is a good "how to", as well as a nice gallery and the "C.O.W.", or Carving of the Week. Very nice! The instruction page is written by a non-artist, so there are lots of tips on working with copies. The transfer tips could be especially helpful.
The consensus is that pink rubber and art gum erasers aren't worth the effort, and I agree: the best pink or gum eraser won't work as well as an inexpensive white vinyl eraser. (Magic Rubs aren't as good as Staedtler Mars erasers, but you can still get good results with them, and can usually buy them by the dozen at one of the office supply superstores.) Just make sure your eraser doesn't have any raised areas (many Staedtlers do). If it does, it's unlikely your design will allow you to carve all of it away, and it will show up when you stamp. I found this out the hard way-I'm glad it was a quick, easy-to-carve design.