With the canter being as dynamic as it is, 'cantering more' does not improve the canter. Jean-claude racinet (sp?) said to a horse, there is no 'good' or 'bad' canter--they only give you the ONLY canter they can at the time, given their current state of fitness and balance. Philippe Karl says that you can't really improve the canter, from the canter--the canter is a test of balance for all gaits. If she has bad balance at the canter, chances are, she has bad balance in the other gaits too--they're just naturally more stable so you don't realize it.
Start doing transitions, and build on them. Don't go asking for walk-to-canter transitions, she can't do them yet! Walk-to-trot is good, walk-to-stop. Trot-to-walk, eventually trot-stop. You're not trying to chase her in and out of the gait, you just want the transition to be smooth. Don't hold onto her head when you ask for the transitions, let her find her own balance. Making a pretty big circle, ask for at LEAST two transitions in one rotation. LOTS of transitions build up your horses topline, which she'll need to canter. Start doing leg yields and shoulders in at the walk and trot--by loading all of the weight onto one back leg seperately, it begins to teach the horse to take more weight on both legs when traveling straight.
Remember, the canter is a measure of your horse's progress, not something you just keep doing to improve! If you have any questions on the exercises I suggested, just ask. ;)