1. Reactivity often starts in the home, so when fido is home alone, make sure he isn't left in a room with a large window or door that he can see passing dogs or people, if he is home all day barking at things this heightens his stress levels for walks and also allows him to practice bad behaviours. This includes if your dog lives outside, you need to find a way to either section him off to only part of the yard or block his vision to the street with some tarp, some hedging or a fence that is solid.
2. If you don't have to bring your dog in the car then don't! Dogs barking in cars is another big contributor, if you do have to bring your dog, consider buying shades for the windows to pit up while your gone or leaving your dog in crate if he is crate trained and place a blanket or cover over it so he cant see outside
3. On the walk, tension on the leash = reactivity! When you see the triggers that make your dog react, don't suddenly wrap the leash tight around your wrist and start tugging on your dog. Try and keep the leash loose, take a breath and talk cheerfully to your dog. If you can do a calm U-Turn and go the other way while keeping a cheerful voice, forcing your dog to walk past his triggers only makes it worse! Take a another quieter route or go walking at quieter times of days to help your dog feel more relaxed!
There is much more to it of course, but these management steps can help keep your dogs stress levels low and make life easier!