Very good posts above. One more thing you may like to try is a clicker. The beauty about this method is that the clicker is a sound that the dog only hears when she gets a treat. Using 'watch me' commands are good, but sometimes they can be ignored easier than a clicker.

Basically, as soon you see another dog, click the clicker and give her a treat. Obviously you need to do this before she gets worked up. To start, you'll need to 'load' the clicker's sound. So, at home, practise some basic obediance and whenever she does something you ask, like 'sit' or retrieves a ball etc., click and treat. Use high-value treats to build the association that click means a treat. You are effectively marking good behaviour.

From then, you can build positive associations with other dogs out on walks. It works in a number of stages: 1) When she sees another dog, click and treat 2) When she sees another dog, click and wait for her to look up at you, expecting a treat- then give her one 3) Then, she'll eventually look up at you automatically upon seeing another dog, which you click and treat for 4) Remove the clicker and just treat when she looks up 5) Phase out the treats, so she gets 1 treat for looking up at you twice, instead of once, then every three times, she gets a treat etc. Substitute treats for praise.

The above stages need to completed one at a time so that she can do each consistently before you move to the next stage. You can begin to bring in a 'watch me' too around stage 3 or 4, if you like.

Hope this helps. It sounds complicated but it really isn't. It's my favourite method when beginning to work with dogs who are reactive or nervous in different environments.