Focus on Reflection vs. Content
Mirror Speech gives a good example of true improvisation. It is not turn-taking or mimicking. Rather it allows both players to explore ideas and let the story (such as it is) go where it will. It will often become a flight of fanciful ideas. Listen to the explanation Viola Spolin makes after the exercise.
Things to Help Mirror Speech
Avoid lists – Old information, mentally treading water, regurgitating what one already knows, keeping the conversation ‘earth-bound’ and makes flights of fancy harder.
Avoid questions – other than rhetorical questions it is not a dialogue but a reflection
Avoid using the word “I” – Again, Old information. It makes for separate opinions (subjectivity) and keeps the players from merging into one voice. It will also keep the players ‘earthbound’.