A persuasive essay is an essay based around the purpose of convincing the reader of your point of view. Persuasive essays are usually based around an argument, especially an argument which is contrary to the accepted opinion of society - you may argue, for example, that a healthy diet does not have to contain fruits or vegetables. The more widely accepted the topic you are arguing against is, the harder it will be to persuade; however it does mean that you will be challenged and therefore produce a better piece of persuasive writing.
An absolute necessity for a persuasive essay is being clear about what you are arguing. You will simply not be able to achieve the purpose of your essay if you are not clear yourself about what you are arguing. It may be necessary to do some research on the topic if you aren't confident with the topic, but try to choose an essay title that you have some experience or prior knowledge in.
You should also make sure that you know exactly who the audience of your piece is. You will need to understand the audience to engage with it - if you are writing to school students, you will have to tailor your language differently than if you were writing to a local councillor. The audience for your piece will nearly always have the contrary view to what you are arguing (as this is the foundation of the essay), but persuasive essays can also be used to reinforce an idea to an audience sympathetic to your view.
To make an essay effective, you should identify your strongest points before writing. Have a clear plan in mind before you start writing, so that you can spend your essay writing time focusing on the style, rather than content. Similarly, you will need to identify the contrary points to your view, so that you can write about the flaws in the opposing view - counterargument is a great way to effectively persuade in this sort of essay.
Once you have your points, the structure of your piece is fairly simple. The introduction of the essay should catch the attention of the reader, and potentially outline your viewpoint previously. From there, it is a good idea to write each point in a separate paragraph and conclude with your strongest point. If you counter-argue throughout, you’ll succeed in writing a persuasive essay.