Keep your canter aid the same every time. Sit back, slide your outside leg back. Very simple. :) If you ask for the canter in the same couple of spots that will help her out as well because she'll have a simple yet clear pattern. Ask for canter, and if her trot speeds up slow her down and ask again. Not more aggressive, just the same. In a few transitions she will start to look for a different answer and she might canter. Try not to let her speed up and rush into canter, as you mentioned. :) Pay special attention to her balance also. If she loses her balance in the canter aid, abort canter and get her balanced again. Once she picks up canter and you feel her attempt to slow down or fall out of it, allow it. Allow her to canter for as long as she stays balanced and can canter with ease. If she falls out, re-balance and ask again. It will only give you more opportunities to get more transitions in. The more you repeat, the more she'll start to organize herself sooner and stay in canter for longer. It is really important to remain balanced yourself. Try not to let your upper body tip forward, stay back so she can balance up underneath you.
A lot of times I will cluck to them in the rhythm of canter. Change the rhythm to canter in your seat. Your whole self portrays canter. Any effort on her part to change smoothly into canter she gets big rewards. :)
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.