Before you start working on your essay, make sure you know all the rules. Do not test the patience of your readers by breaking even a minor rule. When it is possible to do so, get the latest set of rules from the organization's web site--they might have changed since the last time you checked--and look up the rules again before you submit your essay.
Basic rules to watch out for include the due date (sometimes the deadline is extended), the length (maximum or minimum words or pages), margins and fonts (use standard ones if there are no explicit rules), and the title (sometimes a certain title is required).
Most importantly, make sure you very clearly understand what you are supposed to write about. If the prompt reads, "Account for your opinions about the role of the United States in global warming," that is exactly what you are supposed to do. It would be wrong to frame your essay around global warming. It also would be wrong to frame your essay around the U.S. contribution to global warming, or even the U.S. role in general (in some ways contributing to it, in other ways reversing the effects). It even would be wrong to frame your essay around your opinions about the role of the U.S. in global warming. Indeed, the prompt asks you to do none of those things. You are supposed to account for your opinions on that topic. The real topic of your essay might be, for example, a book you read, a television show you watched, or a news report that led to your opinions about the U.S. role in global warming.
Likewise, if you are required to include certain themes or sources in your essay, do not fail to include them.