At school, we had a weekly English class in condensing text. It was in the days of O-levels, OK? They also made us read Hardy which is really uncalled for. I was rubbish. I’d submit stories in creative writing that were 1800 words instead of 800 (a Marlowe pastiche – Philip, not Kit). Then I discovered the drabble: stories told in exactly 100 words, no more, no less. You really have to condense to make 100 words tell a story. Cutting adjectives, finding tighter phrasing, understanding colons. All of those came from drabbling. I recommend it to people as good practice.
Tonight I found I have a new favourite editing warm-up. 140 characters, inc. punctuation. If you want to learn to edit sharply, get twitter.