I already have advanced surfer's ear so prevention is no longer an option, but the doctors i have seen have told me that they think the two primary causes are:
excessive exposure to cold water in the ears.
excessive exposure to cold air and wind in the ears while surfing.
I didn't like Doc's Pro Plugs, which are popular among a lot of people, so I use Macks silicone putty plugs to keep the water out. I pretty much wear them year round every time i surf.
I also wear a bonnet hood, skull cap (whatever you want to call it) to keep my head and ears warm now that air and water temps are below 55. I've got an O'Neill hood that i like a lot. Obviously built in hooded wetsuit once it gets a bit colder.
These two practices seem to keep the cold water out, which is helping reduce the symptoms of having surfers ear which are 1) loss of hearing 2) infections both due to the water never draining out.

