Establish a relationship with the best Vet you can find. Depending on the overall condition of your dog it may be advisable to have it checked every six months. Your Vet should be someone whom you trust and with whom you feel very comfortable.
Educate yourself about the conditions common to older pets and the therapies used for them. Be alert to symptoms so that you can bring them to your Vet's attention promptly.
Feed your older dog the best food you can afford. Consider feeding two small meals daily rather than one large one.