In my years of battling ich, the only thing that ever truly worked for me was to add lots of cleaner shrimp and to help the fish kick the ich themselves. As others have stated, ich is always there but it rears its ugly white heads when it finds a host that is weak and stressed out. Fish get weak and stressed out for many reasons. I usually get ich outbreaks when new fish are added. Even some of the existing population develop ich because they stress out about the new addition. Sometimes one or two fish get ich if I have left a water change for too long. Generally if they are healthy they will kick the ich themselves. The cleaner shrimp help them along by picking off their ich. I have 4 skunk cleaners and 4 blood red shrimp and all 8 of them have cleaning stations that the fish line up for when they get ich, or even if they are just itchy. With 8 shrimps, I also get a lot of shrimp spawning.. they are a busy bunch!
Anyway, that is my strategy and (thankfully!), even though I get ich now and then I have yet to lose anyone to ich (since adding my population of shrimps a couple of years ago.
K.