My 64-or-so pound dog once had a cough/respiratory infection in the middle of winter, and the vet told us to wait it out. She was coughing a lot and not doing well, so I started some home treatments for her. I made sure she was warm, and added some powdered garlic and water to her dry kibble each meal. I also gave her, several times a day, a mixture of about 1 tbsp lemon juice, 1 tbsp honey, and added some to her kibble. It really seemed to soothe her throat and keep her from coughing. To encourage her to eat it, I had to add crushed biscuit.
I also added wild rosehips to her diet. They have a huge amount of Vitamin C, and I would add handfuls to her food and give them to her as treats. She consumed a full quart of them within 1 week! Her recovery was quick and she was soon back to her normal self again.