There are a few important rules we follow:
1. We can't criticize or make fun of ourselves
2. We recognize that 5-20% of the movement is going to work beautifully and the rest is probably not going to be as useful
3. We know that on some days the muse will be with us and on others she won't be with us - and this has nothing to do with our worth as dancers
4. We are willing to be vulnerable and share the improv with ourselves and others
The one thing that will kill creativity and shut off all hope for interesting movement in improvisation is judgment. It's a bit like mindfulness meditation - the idea is not to place value on the moves that you bust out - but to just accept them as moves. They are not good or bad, they just are what they are. This lets us go deeper and find more inside us. We're going for spontaneous fun--letting go and being in the moment. The minute we decide that we look stupid or put value on our movement we kill it. We go into our heads and out of our bodies.
The cool thing about doing it with my son is that it holds me to the rules. As a mom my instincts make sure that we are creating a judgment free zone. If I start to criticize myself...I catch myself. I don't want him to see me putting myself down or feeling like a dance impostor. I don't want him to do the same. Yet, I might not do that if I were working alone.
Sure, some moves will work better in some flows than others - and figuring that out is part of the work -- part of the process. But we will kill the process completely if we start judging ourselves when we are at our most vulnerable -- working from the spirit instead of the mind.