In my experience, it's best to clean as soon as possible, to prevent further drying and caking. That said, when I do have a caked on situation, I start by filling the carboy with plain water, a long soak works wonders. If I'm in a hurry or the previous step wasn't enough, I have added bleach. This always works. I recall using about 3 capfulls. Make sure when working with the bleach solution that you put on work clothes. When you pull out the carboy brush, you can't help but spray bleach-water all over the place.
Good luck