An instructor at pony club told me once to visualise and ride circles in 4 quarters, so if you stuff up one quarter you can fix it up in the next, which makes it a lot easier than trying to fix the whole thing :)
My usual instructor has a cone in the middle of the arena with some string tied to it for drawing circles. He walks the string out to however big the circle is going to be and just walks in a circle holding the string and scraping a track in the sand with his foot. It's really handy when you're doing a big 40m or so circle which are much harder than a smaller circles because you can't see the whole thing perfectly haha :P hope this helps :)
Eventing, the sport where you strap your medical information to your arm.