I've posted this link before but who among us couldn't use some leash-walking tips? There's a good video and 7 handy tips for happier walking, whether the issue be pulling or aggression. Also at the bottom in the 'related posts' are some really good pieces of advice about dealing with dogs that get aggressive on the lead.
I recommend looking at this one first, with video:
and then down at the base of the post in related posts, there's a good list of some further posts too. 'There's always a sign' is very good. I know many of us have a dog that may get really riled when the dog meets another dog on a walk (Lily is like this) and managing encounters, and improving behaviour, makes walks so much more pleasant!!