We've actually had this thread in the recent past - so I'll say this again........
The BIGGEST, NUMBER ONE, do-this-if-you-do-nothing-else thing we can do to have healthy dogs is to maintain them at optimum or even slightly thin weight!
Other tips:
Do regular routine "well" full blood chemistry panel with CBC and urinalysis. This is so useful in detecting problems early, before symptoms, when you have a chance for early treatment that can make a huge difference as to outcome. This is especially important as our dogs get to middle age and older.
Find a GP vet that can hear soft heart murmurs. Take chest radiographs when murmurs progress to a middle grade or at middle age even if murmur free so you have a baseline for comparison when disease progresses. (My definition of middle age is 7 to 9, senior is 10-12, and geriatric is 13 and up. I realize that others have different definitions.) Best scenario is that a Cavalier owner should know the status of their dog's degenerative valve disease long before there are obvious symptoms. Surprises are never good.
When necessary, work with a good specialist. Don't wait until you are in a crisis. Huge difference in quality and quantity of life. If possible, assemble a quality team of GP vet, any specialist that you might need (cardiologist, internist, neurologist, ophthalmologist, TCM/acupuncture vet) and remember that you are the central member/coordinator of your team. I realize that this is something that will take some years and is not instant for a new Cavalier owner and may not even be possible depending on whether you live in an urban or rural area. When you find a good team, treat them well! Be respectful, remember the vets and clinic staff with lunches, baked goodies, etc. a couple of times a year.
Make a chart/folder of your dog's records and keep in your home office. Get copies of test results, reports, blood chem panels, etc. Make a list of your dog's meds, history of vet visits, etc. I even keep all of my dogs' radiographs in my home office. This is of huge benefit if you have a consult with a specialist or you have an emergency room visit. Not only will the new clinician have an accurate history on your dog, but you will gain instant respect as a responsible guardian. Also, in a crisis, we tend to forget important information because we are upset. I can walk into an ER and hand a complete chart to the clinician. Take some time to find specialists, 24 hour hospitals, etc. in your area before there is a crisis. Talk to other dog owners about their experiences with different facilities. I give more credit to "my" specialist team than to any other factor for the longevity of my Cavaliers (ten past and two present).
Become educated about problems in your breed. Find credible sources of information. Print the materials and keep them handy. Perhaps purchase a veterinary textbook about something of particular interest about which you want to learn more. (I highly recommend 3rd edition of Manual of Canine and Feline Cardiology - available on Amazon - not too expensive. Also get a copy of Plumb's Veterinary Drug Handbook.) Learn how to understand the results of a blood chemistry panel.
I am not a fanatical diet person - there are several good alternatives so find what works for you and your dog. But do not feed cheap grocery store kibble. If you feed kibble, find one with decent ingredients - there are many. Be skeptical of veterinary prescription kibbles and learn about alternatives.
I am less gung-ho about supplements than I was in the past - but fish oil capsules seem to be highly recommended by my internist, cardiologist, TCM vet, and other people that I respect. I'm about to switch from Co-Q-10 to ubiquinol from new info I've learned.
Limited vaccinations - learn about this from source material (such as papers by Dr. Ron Schultz or
http://www.critteradvocacy.org which is a site created by a vet in Texas) rather than from opinions on internet message boards. Find a vet that will agree with your views on vaccinations.
Pat