Know what your argument is before you start writing. No matter what the assignment, the best way to write effectively is to know exactly what you want to say long before you sit down to type up your thoughts. To accomplish this, prewrite or brainstorm your ideas on paper, or even just talk them out with a classmate until you've developed a working thesis.
Write a rough outline that begins with a thesis statement. A thesis statement is an argument that you will prove--and you must know exactly what this is before you can proceed. Three to five facts or examples that support your thesis must follow next in the outline. The last item must be a conclusion that essentially restates the thesis.
Write your first paragraph. The first sentences should be an interesting introduction that present the problem the persuasive essay will discuss. The last sentence of the paragraph should clearly state the thesis.
Write a handful of additional paragraphs--at least three--that support your thesis. If you are making an argument about a novel or poem, you can select quotes from the literature that support your argument. If you're discussing a political controversy, choose reasons that bolster your claim, and find relevant quotes and statistics to cite.
Address any counterclaims in the body of your persuasive essay as well. For example, if your argument is in favor of capital punishment, refute commonly held beliefs about why capital punishment is either morally wrong or not effective. Counter-claims might not apply in a literary persuasive essay but usually does in one about politics.
Conclude your persuasive essay with a paragraph that sums up the evidence presented and restates the thesis.
Carefully proofread and spell-check before submitting your persuasive essay. Also ensure you've properly cited sources using MLA or APA style, whichever your professor requests.