Reactive parenting is like an airplane on auto-pilot flying off-course: to set a new course you must first get control of the plane on its present course.
That means, paradoxically, that you can avoid an unwanted reaction by deciding to do it deliberately!
Example: You're about to yell at your child, and you can't stop yourself. So you decide to yell on purpose, with the full force of your deliberate intention! BUT...
You alter the outburst in some way that makes it benign, like singing a really high/loud note instead of yelling, or changing your words to "Blabbety! Blabbety! BLAH!" Anything that works with your emotional energy rather than against it.
Once you've shifted from REactive to PROactive, you can direct your energy consciously, in accord with your ideals.
This technique is not a panacea, but it works especially well with mild reactiveness — often turning potential conflict into playful fun!