Try squeezes with your inner thighs. Keep your legs long, squeeze and roll up and out of the saddle (as if you are lifting yourself with your inner thighs). Not sure if that is a good explanation. You should feel like you are controlling the posting and keeping with the movement smoothly. I don't ride dressage, but I just want you to experience what it feels like to be controlling the posting first.
If someone knows a better way, speak up :)