Some competitions involve not just an essay or two but a resume, an additional writing sample, and even an interview for finalists. Think ahead of time about a potential interview. Will you be able to defend and corroborate the information in your essay? Will you have something worthwhile to say if someone asks you further about the experience or about the research you did? If someone presents an argument that goes against your position, will you be able to respond? Can you restate your argument from memory? Could you speak about it intelligently for two or three minutes if necessary?
Even if you are not going to have a formal interview, you will benefit from considering these questions. Imagine somebody interrogating you about your topic, presenting counter-evidence and counter-arguments. Make your essay solid enough to withstand the pressure, and show that you have considered alternative positions.
