Heartworm disease is a very real risk for dogs. For more information, see my post at Heartworm disease . There are a number of effective ways to prevent heartworm disease, both holistic and traditional.
The most 
important preventative measure is to keep your dog’s immune system strong with an appropriate diet and decreasing the exposure to toxins. Vaccinations and all traditional medications, including antibiotics and pain medications can cause toxic reactions in the body that will decrease the efficacy of the immune system. See my post on minimizing vaccines in your pet. There are many reports of this approach alone proving effective in preventing heartworm disease in dogs.
The traditional method of prevention consists of a medication given in a chewable treat once a month during the mosquito season. These are effective, but come with a higher risk of side effects and adverse reaction. There is also no guarantee that a dog on a traditional preventative will be protected from heartworms.
There is also a natural alternative called nosodes. Nosodes are a homeopathic preventative made from the diseased tissue of an infected animal. Get more info in my post on Nosodes. Nosodes, in combination with the immune-building steps listed above can be very effective at preventing heartworm disease in your dog.
Heartworm testing should be performed yearly to be sure that your dog has not contracted the disease, regardless of the preventative option you are using.