Are you talking about the entire topline of the horse or just the neck? If it's the whole topline that is lacking problem, the advice I have gotten for Night Heat is to try to get them to work long and low, and get them working from the hindquarters, because that in turn will get their front bit to also work properly, thus bringing more muscle.
Also I have heard hill work works wonders too. But I am not so fortunate to have hilly places available for that. :roll:
Hope that helps abit.