The first and most important thing that you should do before even beginning your essay is to take a moment to do a quick plan.
This should briefly outline the main strcuture of your essay, the key points you want to make, any quotes and themes etc and the order you'd like to set your ideas out in. This should help you to keep your essay focused, enabling you to include all the points that you intend to and will hopefully prevent you from venturing off topic.
Here is an example of how you might set out an essay. Of course everyone has their own style of writing but these are probably the key bases to hit in order to create a strong foundation for any essay:
1. Introduction: This should be concise and focused on the essay question, it should give some context about the text(s) in question, such as a brief description of the setting or an outline of the characters in question, for example. You may also want to briefly state where you are heading in your essay and the main things that will cover. This shouldn't take you long and should only be a few sentences in length.
2. Main Body - Set this out as a series of paragraphs, each following the PEE structure:
Point - state what this parapgraph will demonstrate and be careful not to story tell.
Evidence - the points you are making should always be backed up with evidence from the text. Try to include a fair number but not to the point of overload. If you are making lots of points without evidence or giving lots of quotes without making a point then the examiner may question your understanding of the text, and we don't want that. If you can't remember a quote for every point, then don't worry, you can always brielfy desrcibe the moment instead.
Explanation - After making your point and providing some evidence, you must then explain the effect of the evidence that you have quoted. Try and critically analyse it. These questions may help: What does it show? Does it particularly emphasise something in the scene? Does it emphasise a major theme of the text? What effect does it have on the reader? Are there any literary devices that are being used that may aid the point the author is making?
At the end of each parapgraph, try and remember to link it back to the main essay question. Again this will prevent you from going off topic, keeping your analysis focused and will show the reader that you have really considered the essay question.
3. Conclusion - An essay without a conclusion is like a story without an ending. Like the introduction it needn't be too long and the use of quotes is not generally encouraged. Simply outline the main things you have discussed, drawing back on the key points that you have made in the main body, and then link them back to the overall question. This should show that you have carefully considered the question throughout your essay and have been critical in your analysis. :)