I haven't experienced fin biting yet, but I think it's just a process of elimination + clean water to prevent infection. For example, if your filter is quite strong try baffling it to reduce the flow, or, if you haven't moved the tank around much, try adding in a few bits and bobs such as some cheap floating stem plants to prevent boredom. Conversely, if you're changing the tank around a lot, try leaving it a while in case he's stressed. Make sure your water is very clean and your water changes appropriate for the tank size and whether it's cycled or not. To help prevent infection, I believe half an Indian almond leaf once a week releases tannins which are beneficial to the immune system, and soaking food in garlic extract can help too. I've got a half moon on order from aquabid so I've been researching this a bit, and I don't think there's any definite way to prevent it, however as long as you take steps to avoid infection it should be fine. Bettas bite their fins for different reasons... if he has particularly heavy fins he may never stop trying to give himself a trim!