I love having sippy cups that don't spill. What I hate is the small crevasses that they come with. One day I happen to have a plastic paint brush from a kit that the kids had near the sink. So, I wondered to myself if it would get into the tiny spaces and help get the cups clean....and it did! I have been doing this for a while now. Just get the lid nice and soapy and use the brush to scrub around in those tiny places, then rinse.
Hope this helps some out there that have the same problem I had!