Establish a safe word or two - something to let you know if you are getting close to her limit (like "Yellow" or "Mustard") and one to let you know you have gone too far (like "Red" or "Ketchup"). Pick words that would normally not be used in play.
I also agree with the previous comment - be assertive and confident. You are the DOM, so act like it. Otherwise do what you like or even what she likes, as long as you are the one that is permitting it.
Most of all - communicate and have fun!