The key to a good essay is planning ahead. Firstly look at the question and form the answer by putting down the main points, which will then become your paragraphs. Never put down more than 4 main points because you will run out of time trying to explain them.
All essays start with an introduction where you clearly have to establish that you know the novel/poem/play well and that you intend to answer the question and directly as possible. You must do this by writing an introductory sentence where you clearly answer the question eg. In Shakespeare's famous tragedy 'Othello', the theme of jealousy is reinforced in a number of ways, predominantly through striking imagery, interactions between the characters and diction used by Shakespeare. Avoid long lists and aim to only identify the strongest points in your introduction. A well written introduction sets a good mood for the entire essay. REMEMBER TO ENSURE THAT YOU REFER TO THE QUESTION IN YOUR INTRODUCTION.
The body of your essay is critical if you want to achieve a high grade. The golden rule here is QUALITY>QUANTITY as it is much better to write 3 or 4 great, detailed paragraphs than to fit in many small and hardly relevant points. Each paragraph must begin with a sentence which clearly identifies what you will be explaining within your paragraph. This is called a Topic Sentence eg. Shakespeare uses imagery as a key way to reinforce the theme of jealousy which haunts the play throughout.
You would them move on to give examples to justify your point and the main way to do that is bringing up examples from the text and quoting them. QUOTES MUST NEVER BREAK THE FLOW OF YOUR WRITING so never start the sentence with a quote and then explain it; carefully embed the quote into your writing eg. Through the use of the image 'green eyed-monster' Shakespeare grotesquely personifies jealousy which gives it a stronger and more striking presence.
Do not put in too many quotes, instead take care to explain their effect. Some times a quote is not necessary to convey a point and you can simply bring up an instance from the text. Usually 2-4 quotes per essay is sufficient.
Finally you have a conclusion in which you NEVER introduce new points, but simply summarize the points you have made in your paragraphs. Remember that a conclusion is your last chance to show off your language skills to the examiner.
Always remember to be concise, use shorter, simpler sentences if in doubt. Though examiners do not look for flawless spelling, your grammar and punctuation are key to making your writing flow. Make sure you practice timing yourself with essays to ensure you know how much you can write in a give amount of time.