Much of the writing advice I've heard centers around using fewer words. Twitter's 140 character limit exercises the editing-to-reduce-size muscles nicely. For example, here is the scratchpad I just used to write a tweet.
Whittling down a tweet.
Seeing it like this, I realized that editing is to writing what taking multiple shots of a scene is to photography. It gives you lots of versions from which to choose the best one.