I read the thread on small dog aggression and as the owner of 2 chihuahuas, I echo every sentiment. However - with my youngest dog, I was diligent about socializing him, taking him to obedience, doggie daycare and making him "live on the ground". I read alot, I learned a lot, I did not baby him. Still, he ended up with leash aggression. I didn't want to be one of these people with a small dog that is yappy on leash toward other dogs. But that's what happened. Ironically, my oldest dog is a rescue and lived her life in a cage before me and was extremely UNDERsocialized and it turns out she is the better behaved dog - on leash and with strangers...she loves everyone! Yes, a LOT of a dog's personality depends on 'nuture', but I think a lot of the time, they are born the way they are.
Anyway, for the last month - I have been trying to straighten out the bad child.

Our life has been based on the NILIF program. And I accept responsibility because I think even despite my efforts, he felt as though he still ruled the roost. THAT has changed. I brought in a trainer for one day to get us started and she got us off on the right foot. Little everyday things have made him understand that he doesn't need to protect the rest of us (no couch time, sitting before eating, no walking ahead of me thru doorways, making him lie down beside me for 20 minutes at a time). The change in him has been amazing.
On our walks, I started with a gentle leader as I taught him the 'watch me' command and not to pull on leash. I switched to a no-pull harness and when we walk he is beside or behind me and we stop when I want to stop. Not when he wants to sniff every tree. His body language has changed - his tail is no longer all the way up when we are out there. It's in the middle. And he listens to me.
His reaction to other dogs on leash has changed from barking his head off (I think he had little confidence in me as a leader and felt he needed to protect us), to fewer barks, to a DELAY before the bark, and last night I was beside myself: we encountered 3 large dogs (all across the street, because we haven't worked up to that level yet!). But, when he saw them, he sighed, then I distracted him with the "Watch Me", gave him a treat and we kept on going. NOT ONE BARK THE ENTIRE TIME! It was like Christmas!
Having said this, I know we'll have more bad days, but yesterday gave me a glimpse of the good ones and I realize what we are doing is actually working. Yay!
Good luck to you all and Happy Canada Day to my fellow Canucks!