My approach in working with any child and parent is to identify the likes and strengths of the child and to use those to bring about the desired positive changes. We all can get caught up in naming certain characteristics that annoy us about our kids, e.g. "oppositional behavior", and treating our kids as if they have some "disorder". My profession, unfortunately, has been the leader in convincing parents that virtually every persistent (and even transient) problem they are having with their kids is cause for great concern and all too often, cause for medication.
What is really going on with your child when she occasionally doesn't want to drop what she is doing and do what you want her to do? Let's view this a little differently. Let's view this as her being so involved in an enjoyable activity that she doesn't want to stop it, not as the psychiatriic term, "oppositional response". If we are looking to identify your child's strengths, why not look at her control and stubborness as "strengths", abilities to become very involved in paying attention to something.
Focusing on her need to control, you can find lots of situations where you give her "natural" control, like telling her she can come get you now or in a little while to do something special with her, asking her whether she wants a certain light on or off in the room when you read her a story, offering her the choice of selecting which pleasurable activity she wants to do now--you get the idea. Between doing these "dress rehearsals" of transitions at times when they are not really demanded and finding many opportunities for her to "flex" and develop her healthy controls, you can begin to take your requests out of the arena of conflicts and into the arena of communication that happens all the time involving choices she makes.
Good luck, Cathy; it's always a pleasure talking with a loving parent like yourself.