I was struggling with keeping my stirrups at the canter for a little while. It was mostly because I was twisting in the middle - I'd let my right side get a little dippy and lose my right stirrup. Improving core strength will really help you. I regularly practice riding (at all gaits) without stirrups; it helps me be more aware of my positioning. You can't tense up, you have to have a nice circular or eliptical motion with the horse's rythm.
This video helped me visualize my positioning and my own movement at the canter: