This might not be news to anybody but it was to me and it worked great. I had a carboy that I couldn't get clean, even after soaking in PBW for a few days and I couldn't get my brush to reach. I read this somewhere and thought I would try it out.
Add a cup or two of rice to your carboy with just enough water to make a slurry. Swirl it around vigorously and the rice will scrub off any junk especially in those hard to reach spots. Simple and effective.