Write your persuasive essay with the reader in mind. Since you are trying to persuade the reader to agree with something, it is essential that you understand the frame of mind of the reader. Consider an essay on the benefits of walking as an example. Rather than preaching at the reader about obesity or fitness, begin with a description of the pleasures and satisfactions of walking.
Accept that there are some readers you will not convince. Rather than addressing those readers who might be adamantly opposed to the point you are making, focus on readers who are undecided or uninformed about the issue. Focus on people who are looking for low-impact exercise rather than members of the local car club. There is a much greater likelihood that you will be able to convince these readers of the validity of your argument.
Brainstorm by writing down all of the arguments in favor of and against the point you are attempting to make. If you can address and counter the arguments that are opposing your own, this is an effective means of convincing the reader of your own authority and authenticity. Anticipate the argument that walking somewhere rather than driving takes too long, and consider arguments such as the time it takes to make the money to run the car.
Respect the reader's right to disagree with you. If the tone of your paper is one of browbeating or arrogance, you will not convince anyone of your point, even if it is valid. Prove this to yourself by reading persuasive papers written by other people. Find examples of well-reasoned papers, and hysterical, adamant papers. Read them objectively and gauge your own reactions.
Back up any statements that you make with facts and references. How strongly you feel about the issue has nothing to do with how valid it is in the reader's mind, so don't preach or harangue. For the walking example, there are reams of evidence that can prove the health benefits of regular exercise.
Conduct solid research before writing your paper, and refer to trusted and respected writers to validate your claims. Making reference to other works that are generally recognized as trustworthy is an excellent way of increasing the persuasiveness of your writing.
Construct your argument in a logical progression from beginning to end. Finish your paper with a brief and comprehensive summary that leaves the reader with a solid grasp of the issue, and a believable reason to agree with the point that you are making. For example, a) walking has proven health benefits, b) cars and traffic are unhealthy, c) you are interested in being healthy, d) therefore it makes sense to walk more.