Something else you may want to try: Strongid C or Strongid C2X for a month or so (doesn't take care of everything though, so make sure you use something broad spectrum as a paste dewormer, like Equimax or Quest Plus).
I second cafl! Daily dewormers have a huge benefit over monthly or bimonthly wormers, as they do not allow the worms to mature and cause damage to your horse's internal organs. I was sick to my stomach when I read that our "regular" deworming schedule still leaves the horses with worms crawling around in them and puncturing little holes everywhere until the next deworming.
I like Equiaid CW because it is cheaper than the Strongid. Or, I remember another forum member (amberp) saying that she uses diatomaceous earth as a natural daily dewormer, as it kills worms and fly larvae. I'm thinking of trying that myself.