Theophylline is another option either instead of, or in addition to one of the non-steroid preventatives. Accolate, Singulair, or Zyflo may help if there is an allergic component to your asthma.
Atrovent is another option, but would probably need to be taken four times each day to work as a preventative.
I will let you know if I think of anything else. Good luck working out some kind of treatment with your Pulmonary Specialist. I hope you find something that works for you!