Good question but it raises another question, ie., what would you like to prevent in your dog?
Well, for most diseases like rabies, leptospirosis, distemper, parvo, etc timely and regular vaccination is the best preventive method,
In addition you should take care of the following to ensure a healthy dog which you would be proud of ;
1. Good nutrition (ask your vet)
2. Plenty of exercise
3. Regular grooming
4. protection from ticks and fleas
5. regular de-worming
And above all, Lots of loving care!!!