Is it the rising trot you are struggling to keep her going in? If I get a lazy horse I use the sit movement to put my leg on, instead of just letting my bum fall back to the saddle I wrap my legs and use them to pull me back down, this has the effect of giving a forwards cue each stride. I totally agree about getting your horse moving forward off your leg and like Skib I don't expect to have to ask every step but to cue once until I cue for something else, but it isn't always practicle on beginners RS horses too have them so responsive.