Having a crop or over an under is an idea too, but be sure not to over use them, some horses don't respond well to them in some cases. I personally use an over and under that has a little zip to it and it works great for my mare.
I would suggest working on speed changes with control. Ask her to do a circle at a walk, trot, lope and then ask her for some more speed. Don't be afraid to let her open up a little bit in the arena doing this. Make sure first and foremost you have control at all times, if she starts to realize you don't have control at speeds she will more than likely start to test you. Try and see how that works for her and let us know and go from there! :)