Leash Reactivity

It's hard to miss a leash reactive dog. One that appears to be the tazmanian devil, spinning, pulling, barking, snarling, lunging at the end of the leash.  It's disheartening for any owner to see their sweet dog turn into cujo at the flip of a switch.  The object of reactivity can be anything from another dog, a person, a bicycle, a car, anything on wheels etc.  
Some common reasons why dogs react when on leash are:
     -they may feel trapped and unable to run away from something  
        that frightens them so the best defense becomes a good offense.
     -some may have had a traumatic event on leash
     -some may lack proper socialization
     -it becomes a learned association, they see the object and their    
       leash and collar become tight and uncomfortable so they 
       associate that object with the uncomfortable feeling.
     -emotional arousal (excitement), combined with the frustration
       of not being able to see what they are excited about because the 
       leash stops them.  
So what can you do about it.  Progressive desensitization.    Learning your dog's stress signals so that you can keep him under threshold (before he starts to react).  Keeping him at a distance away from the object, before he reacts and reinforcing the lowest level of reactivity with treats.  Dog sees object at a distance, dog gets yummy treats.  This is essentially changing his reaction of frustration, fear or aggression when he  sees the object to one where he sees the object and good things happen...he gets yummy treats.   Start at a distance far enough away that he doesn't react and work your way up closer as he becomes more comfortable. If your dog blows up, you have moved too fast, too soon.    You need to manage the environment  so that he doesn't get the opportunity to practice the reactive behaviour.  As they say, practice makes perfect and leash reactivity is not something you want to perfect!
Teach basic obedience cues such as sit, down, stay, focus, reward mark and no reward mark.  This not only increases your communication with your dog, but it also teaches impulse control.  Start with no distractions and work your way up to the ultimate distraction.  Just make sure you work at your dog's pace. Don't expect him to respond to your cues at the highest level distraction if he doesn't respond reliably at low level distractions. 
Teach the focus command with the ultimate goal of an automatic focus. The automatic focus is where your dog sees the object of reactivity and automatically looks to you instead of reacting.  
Teach a plan B.  We all know that you can only manage the environment so much to prevent the reactive outbursts. Teach a U-turn for when life happens and you weren't prepared for the curveball waiting for you around the corner.  Just like the any other command training, the U-turn should be perfected in low level distraction environments prior to expecting it to work in an environment with higher level distractions.  
In summary, to work with your reactive dog you must manage his environment so he doesn't have the opportunity to practice reactivity, and begin a training program to help reshape his attitude to what sets him off.  Teach basic obedience cues in no distraction environments working up to higher levels of distraction. Perfect the focus command and the U-turn as a plan B.  All these tools will help you gain control over your reactive dog.