Circular breathing is one of those things that just 'clicks' eventually. It seems to help if you concentrate more on the letting out the air part (needs careful control)& let the breathing in take care of itself (just a quick indraw of breath).
I am beginning to get the hang of it, and learnt by imitating more advanced didge-players. It seems to be one of those things that falls apart if you think too hard about what you're doing.

Kris
PS - cats LOVE didges, they get all excited when they find out we can purr too.