Since we had a crack develop in ours over the winter, we just replaced ours. Both pieces were very cheap from the manufacturer (under $15 each inside and out). It's amazing how much brighter it is in the bathroom. I didn't realize how dark it had become. Well worth replacing, and not all time consuming either.
I'll chime in for using the Mr. Clean sponge. I bought the mop one, which comes on a long handle. Let me tell you... no scrubbing or elbow grease needed. It is such a simple solution, we wonder why we ever did all of that scrubbing. LOL