Write about something you (really believe in). If you have to take a stand on a topic you don't care very much about, then try to imagine which side of the argument you'd rather be on. But if given the opportunity to choose any topic, write on a topic what you're passionate about, a topic that informs your life, makes your blood boil, or your conscience too.
At the same time, look for a topic that has a lot of depth or complexity. Your life's interest may be pencil lead, but there aren't persuasive opinions to have about pencil lead. A subject that you're still interested in but which has a lot of depth — like animal cruelty or government earmarking — will make for better subject material.
Begin to consider opposing viewpoints when thinking about your essay. If you think it will be hard to come up with arguments against your topic, your opinion might not be controversial enough to make it into a persuasive essay. On the other hand, if there are too many arguments against your opinion that will be hard to debunk, you might choose a topic that is easier to refute.