The trick is, as GroovyGeek correctly pointed out, staying succinct and on point. How much time did you spend setting up background info for your point? Is that background info actually necessary or does the essay work without it? What is your point?
That last one is a biggie. What is the message you are actually trying to communicate? Try to edit your writing with the answer to that question in mind. Also, is your point too broad? Sometimes you have a great idea, but you are trying to say too much; it may be possible to be a little more specific with your message, write less, and still achieve the same impact.
Final (dangerous) caveat: if you have something REALLY REALLY REALLY important to say, take the extra space. Please use this ability extremely judiciously. Extremely judiciously, I say again. But there are times that family circumstances, for instance, have an impact on everything else, and it isn't possible to explain that piece in 200 words. Understand, however, that a 500 word short answer is only merited in relatively extremely circumstances.