It's strange that such a good topic as this is ignored for a long time......
Of course you have to gain certain knowledge in different fields in order to be well-prepared for every question. The only way to improve this is to read as many books, watch as many English channels as well as listen to English tapes as you can. READ BOOKS THAT YOU LIKE, WATCH CHANNELS THAT YOU'RE REALLY INTERESTED IN or you won't get any benefits...... Anyway, You should try to create an interest in things that maybe you are normally not ( it's a good way to learn different subjects well, too, but this doesn't work with me, lolz ).....
The keys to your essay are organization and creativity ( in my opinion). Organization is MUST-HAVE. You should make an outline carefully, identify what're main ideas, what're their supports and use each main idea with the right supports in one paragraph. That is to make your essay clear, easy-understood. The teacher will definitely give high marks for your organizing skills. The other is creativity. Well, why be creative??? Imagine a teacher has to mark hundreds of essays a day ( kind of boring, isn't it?), What kind of essay do you think he/she is looking for? You've got the answer. They will take extra interest in essays that are really different, really "personal". Yeah, he/she can hear the true voice of the writer. So when writing your essays, try to USE YOUR OWN WORDS, EXPRESS IN YOUR OWN WAYS, AVOID USING TOO COMMON IDEAS AND SUPPORTS....
Anyway, i'm still trying to write good essays ( I'm really bad at this >__< )...... Hope this topic will be continued so that we can truly discuss.