well Berghia are the best route but good QT practices help as well with QTing any new live rock, corals you can get them before they get in the display peppermint shrimp are good as well.
Like Mr. reef said a berghia nidibranch will eat all of the aiptasia, but will starve to death after there all gone. There are also several butterflies that will help keep aiptasia under control but not totally eliminate them. I have had good success using blue life aiptasia control and joes juice. They can both be found at marine depot.