If you are writing a demonstrative essay for a class, consider the length requirements of the essay set forth by your teacher. Choose a topic that allows you to meet the length requirements for your essay. For example, writing about how to tie a shoe might make a good demonstrative essay for a length requirement of one page, but would probably not be a good idea for a length requirement of five pages. In contrast, writing about how to select the best college for you makes a good five page paper, but is certainly not good for a one page paper.
When brainstorming about topics for a demonstrative essay, do not forget to think about the things that you are interested in. Since you will likely have to spend a lot of time with this demonstrative essay topic, it is a good idea to choose something that you will not get bored writing about. For example, if you hate cars, do not choose a topic about how to change the oil on a car. If you like novels, consider writing an essay about how to select a good novel at the library.
Consider the skills you have and the things you are good at when coming up with a topic idea for a demonstrative essay. Remember that a demonstrative essay involves educating your audience about how to do something. If you are not an expert at pitching a fastball, for example, you do not want to write an essay about throwing a fastball in the major leagues. Choose something that you have some knowledge about in order to cut down on the amount of time spent researching.
Think about your audience when choosing a demonstrative essay topic. If you are writing the essay for a class, remember that your audience will most likely be your teacher. So choose a topic that your teacher or audience will find interesting or exciting. For example, do not write an essay about how to swim freestyle if your teacher is your high school's swim coach, since she will likely already know the information. Instead, choose something that your audience will be interested in learning more about.