The Best Prevention Is A Healthy Lifestyle  with the elimination of over vaccinating and chemicals as well as a species appropriate (raw meat, bone and organ)diet.  According to the American Veterinary Medical Association, 65% of adverse drug reactions and 48% of all reported deaths resulting from drug reactions are caused by heartworm preventatives. The potential side effects of Ivermectin (the active ingredient in Heartgard) include liver problems, vomiting, diarrhea, loss of appetite, depression, lethargy, skin eruptions, seizures, tremors, paralysis, autoimmune disorders, thyroid problems, fever, weakness, dizziness, coughing, nose bleeds, difficulty breathing, pneumonia, irritability, sudden aggressive behavior, nerve damage, fertility problems, and sudden death. 
These drugs weaken your pet's immune system which recognizes the chemicals as poison. The body works hard to eliminate the toxins and major organs, like the liver and kidneys, are taxed. 
Part of the veterinary industry has acknowledged these dangers. These "alternative vets" are open to natural, safe and effective alternatives. Many of these vets believe that the long-term use of heartworm prevention drugs are a link in the chain of diseases such as arthritis, liver and kidney diseases, skin allergies, and many types of cancers.
When your dog is on Heartworm pills or chews (all pesticides), mosquitoes will still be attracted to your dog and will still bite them. The key is to get the blood of your pet in a healthy enough condition that the mosquitoes won't want it. Healthy dogs actually have a bitter tasting blood to parasites. Un-healthy dog's blood is more attractive them and may taste sweet to them.
 With proper carnivore nutrition, they will develop and maintain a strong immune system. It is pretty rare for most parasites/worms to kill dogs. They will die eventually when their environment is not conducive to their own life and perpetuation. However, remember, if your dog has a low immune system, the parasites they may be host to are more likely to reproduce and take over with an infestation and could with time kill off their host.
*This kit will last most dogs 6 months or more