Essay writing is an art and every art is learnt, it is seldom a God gifted talent and most of the times it has to be learnt and mastered to perfection. As I said that essay writing is an art so all we can provide is guidelines to be followed, you have to nurture it by practicing as much as you can, but practice in a direction because directionless effort may go awry.

1. As an edifice is erected on strong foundation, similarly the grammatical and sentence structures are the very foundations of your essay. Keep them correct.
   
2. Vocabulary is the way to adorn your expression, felicitous words may make your essay catchy for the examiner. Improve your vocabulary by reading. Sorry, I mean by pleasure reading, it will enhance your knowledge as well as vocabulary bank.
   
3. If you are not good at using good structures and nice words, then keep your text as simple and error free, don't use advanced structures because in doing so you may make mistakes, remember one thing, correct usage of English language is the basic requirement, adorning it is secondary thing, ensure your basic requirement is fulfilled at least.
   
4. Be to the point in your essay, it must not equivocal and misleading, because examiner doesn't have enough time to find his way in a labyrinth.
   
5. Order your thoughts in a natural flow and try to be as much original in your thought as possible. Mark my words, originality of thought and clarity of expression is what matters. Crammers are discouraged, don't use stereotypical material, use your own thoughts.

Summing it up, I would say your essay must have three things, Clarity, conciseness and correctness.

And final advice, Practice as much as you can. Aristotle says "We are what we repeatedly do, excellence then is not an act but a habit".

Best of Luck.

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Ever has it been that love knows not its own depth until the hour of separation.
But when love is one sided, holding off is no more an option.