Depends on the type of essay (text response, persuasive, etc) but:
• Make a plan; even a bad plan is better than no plan
• Write at least 3 paragraphs with 8-10 sentences
• Have a thorough understanding of your text or topic
• Attempt many practice essays and get feedback from the person marking your actual essay. (Also, ask them to mark and give you feedback based on the essay criteria)
• Waste the reading time (if you get any)
• Write about something without proof or evidence to back up what you said.