I totally agree on the assumption most people have no idea how to well maintain or develop a dog. Example, people that have their dog in a house all day to come home given a sligg acknowledgement then let them out for less than five minutes just to release two orifices and run them back inside. Main ingredient for a mildly behaving dog, time spent and getting to really know the animal face to face.
Spending time with your dog one on one and coming to understand just what's causing them to do the things they do (because leash reactivity can be caused by many different reasons, although it's essentially anxiety based) is always a good first step, and will make work with a trainer go a lot quicker if you know the emotional source . Exercise and mental stimulation is ALWAYS a great help, but not a fix to the problem. Now that I'm on a computer, and not my phone, I'm going to post some links that will be of good use here at the bottom.
OP, when Ma'ii and I went to a professional trainer, this is what we did to help curve his re-activity. The exercise is called de-sensitizing and it worked wonders, far better than the "submissive pinning" and alpha dominance bullshit that I've seen ruin so many dogs. The idea of de-sensitizing is to help your dog not be so bothered by the presence of strange dogs, and to focus on you. If your dog doesn't like other dogs, it won't help there. Some dogs just don't like other dogs, just like some people don't like people. Nothing can train that to happen. But you're dog won't feel the need to flip out and react.
First, I brought some treats with me to the training session. When Ma'ii get's into that tunnel vision, it's hard to pull him out of it, and he would ignore everything we shoved under his nose. So I started bringing pieces of slim jims. No dog can resist those fucking things, and it leaves a good after taste in their mouth that they remember. First class took place in a training facility.
I would put him in a heel and we would walk towards dogs that were brought into the room. We would get as close as we could, until he started barking and going nuts. This is called his "thresh hold". As soon as he did so, we'd immediately turn and walk away until he calmed down. The second he calmed down, I'd have him sit, say "YES" and give him a treat and tons of praise. Than we'd try it again, and see how close we could get to his thresh hold. The second time, we were able to get a little closer, and once he reached his thresh hold, we'd repeat the process all over again.
Soon, we started doing this outside in public with the trainer. Dog parks are great for this. We'd never go into the park, but would walk towards the fence and see how close we could get before he started acting up, than turn and do what we did in class. We'd walk back and fourth, around the circumference of the field, etc. All those dogs in motion were GREAT for this!
We still work with our trainer, but do a lot of his re-activity training ourselves now while walking him. He still strains a bit at the leash, but very rarely do we have to turn and walk the other way due to his barking, and a lot of the time he looks back at me when dogs approach as if to ask "I'm doing this right, right? You gonna give me some of that good shit?"
Dogs do follow confidence and leadership, but it's not the same as dominance. You want to be something your dog can look to for leadership, but it should be a relationship that's built on mutual respect, NOT intimidation. You're dog should WANT to please you because YOU are better than anything else. Not because you're manipulating them with fear.
One last thing on the whole "dogs are pack animals and there is only one pack leader". They're not pack animals, actually. They're social animals, meaning they'll chill with other dogs and even work together to accomplish a task, but they don't live together the way wolves do. I guarantee you, if people died off tomorrow, and dogs were left to their own devices, even dogs that lived together as pets would separate and go off on their own. They might meet up and hang out, but they wouldn't LIVE together. If you go to areas with feral dog populations and sit and REALLY observe, you'll see it. It's COMPLETELY different from how it is with wolves. The alpha bullshit thing is associated with wolves, which is what is believed dogs descended from. Wolf packs are made up of a family unit. A mother and father wolf (the "alphas") and their off spring (the pack) which stick around until they themselves are of breeding age and than take off to find a mate. That's it. That's your fucking pack and hierarchy. Parents bitching at their pups to stop fucking around and being annoying.
A lot of it is very similar shit to how people work, which is one of the reasons why dogs and people have had such a successful relationship and been able to live together for thousands of years. Our social aspect is very similar. But we're still a different species, and don't teach right from wrong the same way.
Unfortunately a lot of people still believe in alpha theory in dogs, and are going to try and tell you training your dog any other way is wrong. The thing is, these people are generally (always) extremely ignorant and seriously out of the loop, going of a very old, outdated myth that they remember their parents using on their childhood dogs growing up back in the 1980's and 90's. I think a lot of them mean well, and do care about their dogs, but are just ignorant. A little genuine research and reading up on recent studies will reveal a LOT. But I understand that takes effort, so I'm sure they'll continue to remain ignorant. Kind of like people that still believe the world is flat. Silly.
Here's some links:
On leash reactivity:
http://positively.com/2013/06/18/choice-training-working-with-a-leash-reactive-dog/
http://www.whenhoundsfly.com/on-leash-aggression-towards-dogs/
http://www.spiritdog.com/sanesolutions.htm
Good sites and forums for training advice:
www.dogforums.com
http://www.chazhound.com/forums/index.php
Beyond Ceasar Millan on facebook
https://www.facebook.com/groups/20444826822/
Let's Talk Paws-itive Training on facebook (run by a long time personal friend of mine who is a professional dog trainer)
https://www.facebook.com/groups/514868735245280/
Info on "pack" and "dominance" with dogs (very important for dog training):
http://www.caninemind.co.uk/pack.html
http://www.greatshakesdogtraining.com/are-dogs-really-pack-animals/
http://www.apdt.com/petowners/choose/dominance.aspx
Books:
DOGS by Lorna and Ray Coppinger.
An AWESOME book written by a biologist that conducted a study on the taxonomy of dogs and their evolution. Lots of great info on their psychology and how they work.
http://www.amazon.com/dp/0684855305/
There's sooooo much more out there that can be said, but this post is already a lot longer than I intended. These are all a start!
ETA: Just wanted to add before anyone says other wise, I'm not debating anything here. I lost interest in that back on page 1. Believe me, if I was debating this post would be 3 times longer than what it is, with about 20 more links to back up my case. At this point I only care to help the OP.