There are things such as vinegar, garlic, DE etc, that people believe will help prevent internal and external parasites, but the natural products, beneficial as some of them are to the chickens' overall health, does not help much to stop parasites. See this member's experience...
Deworming every 6 weeks is too much. I personally prefer to deworm twice a year as a precaution and of course, immediately if I see any signs of worms. Here's an interesting discussion on deworming that you can look at...