I have two old Arrowhead Water carboys that sat around my place, mostly outside, for years. And they were already pretty crudded up when I bought them at a barn sale....
I filled them with soapy water and bleach, and let them soak for a couple of days. Meanwhile, I slotted one end of a 2 foot chunk of 1/2" hardwood dowel on my table saw, using a thin-kerfed finish blade. I cut a piece of synthetic chamois about 1/2" longer than the inside diameter of the carboys, centered it in the slot, and drove a couple of small brads to hold everything together. Then I cut the chamois into thin strips on either side of the dowel.
I shook the soaked carboys thoroughly, poured out the dirty water, hit them inside with a high-pressure hose nozzle, then added about two inches of fresh, soapy hot water to each one. I chucked the other end of the dowel into a 1/2" drill motor, and went to town. The carboys came out sparkling clean.
I can't take credit for the basic idea, though. I just built a low-tech version of a cleaning tool I found online, because I was in a hurry and didn't want to buy one and wait for it to show up. My home-made knock-off worked well; I would imagine the original works even better. For one thing, they use a plastic rod that fits in a standard 3/8" cordless drill, which is a little easier to handle than my 1/2" beast. Here's their website:
http://www.carboycleaner.com/
It took me a couple of minutes to get the hang of using this. If your drill RPM's are too high or you don't keep it more or less centered, the chamois tends to wrap around the dowel. And sometimes it'll wrap up for no good reason at all. But it's easy to just unwind the strips and go back to work when that happens, and it beats the living **** out of a carboy brush.