Does your horse also lean in and cut corners when you ride in the arena? They can cut corners and lean because they are very stiff and have trouble balancing on a small arc, or they just want to get the whole thing over with as fast as possible, so are making the circles small, and "pushing" on you, the human in the middle.
If the horse is stiff, then doing work very slowly, such as in this video hhttp://www.youtube.com/watch?feature=player_embedded&v=0mGB4D1Kolottp://www.youtube.com/watch?
Can help. This sort of work is a lot harder than it looks, but the object is the horse following the line, and bending through the body, stepping under with the inside leg and keeping the inside shoulder up.
If the horse is capable of keeping his shoulder up and is just cutting in on you to push on you, use the whip , like Gaming girl said, toward his shoulder. But, start right before he begins to lean in, not after he is leaning. You think of it as "helping" him stay out there on the circle and standing up. So, put some pressure on as he approaches the
place on the circle where there are problems. Get MORE forward energy to help him scoot through that area, and ease off a tiny bet where he respects the circle.