Stay away from personal opinions. You can use personal experiences as an example, but this approach is better for informative essays than for argumentative ones. The purpose of an essay is to inform or persuade an audience, so you want to present the essay in an objective way without showing bias.
Don't use contradictory examples. If you're writing an essay about people's dependence on the Internet and how it hinders students' research skills, don't use an example that talks about the benefits of using online scholarly web sites such as Google Scholar or JSTOR. Stick to the topic at hand.
Use specific and original examples. Your essay will look generic and uninteresting if you use illustrations that others have used or use constantly. If your essay is about how conflicts in interracial marriages affect child-rearing, talk about each spouse's internalization of racial stereotypes as a factor in this conflict and suggest possible solutions to it.