By its very definition, reactive behaviour is an issue with proximity. For whatever reason, the dog at the end of the lead feels that the thing they are reacting to is WAY too close for comfort. This means that the best thing you can do for your dog is to keep safe distance between them and their triggers.
Obviously, this can be easier said than done - we don't refer to the process many of us have to undergo when walking our complicated canines as 'ninjaing' for nothing! There will be times when a trigger emerges upon you and your dog unexpectedly, It happens, and is not something you should ever feel the need to beat yourself up over.
The longer we spend sharing our lives with one of these complex dogs, the more we seem to develop the ability to constantly scan our surroundings and analyse potential issues at an ever-increasing distance. This comes in very useful when we begin to think about using behaviour modification methods, as it means we can start exposing dogs to their triggers at a safe enough distance (although it may be a HUGE distance in the beginning) to keep them under threshold and work on their acceptance of those things.