These are some of the popular preventatives:
My personal favorite is Heartgard/Iverhart. It's a monthly chewable medication. The reason I prefer it is because the protection lasts for more days and it can be safely used for a dog that has heartworms.
Another very popular heartworm preventative is Sentinel. The plus for Sentinel - it also contains a flea preventative. It's also given monthly like Heartgard/Iverhart. You do have to be careful with this heartworm medication. It must be given on time and should never be given to a dog that has heartworms or has missed their monthly tablet.
This last heartworm product is fairly new – Advantage Multi. It is applied to the skin once a month. Advantage Multi does a great job of protecting the dog from heartworms as well as kills adult fleas and is indicated for the treatment of flea infestations. Treatment and control of hookworms, roundworms, and whipworms.
No matter which of these heartworm preventatives you choose - they all give right at 100% protection if you use then as your vet directs.