Lacking a solid answer as to what causes it in the first place, I would recommend all of the following:
1. Keep the tank as clean as possible- the bottom should be regularly cleaned, with frequent water changes (and I do mean frequent--at least every few days or even daily.) Try to provide as clean and stress-free an environment as possible at all times, not just when your fish actually get sick.
2. Treat for Hexamita. Given the frequent association of Hexamita with Hole in the head, this seems like a prudent step to take, and if nothing else, it can help with other infections possibly present. Indeed, most cases of Head and Lateral Line Erosion seem to respond to Metronidazole. (Metronidazole is used in most hole-in-the-head medications or can be readily found in common anti-protazoan medications like Paragon II.) You could also try giving magnesium sulphate (Epsom salts) orally, and raising the tank temperature to 95F (35 C) for seven days. (1) Many commercial products like Hex-a-mit contain Metronidazole, or it is available under the name "Flagyl" to pharmacists and veterinarians.