Pick an item you like–a book, an album, a movie, your favorite brand of gum or cookware, a restaurant, etc. Now imagine that you have to convince someone not to use, buy, or visit that thing or place. Write a compelling argument against your item.
Pick an item you don’t like, and this time imagine that you have to convince someone to use, buy, or visit it. Write a compelling argument for the item.
Pick an item you feel ambiguous about, and practice writing both positive and negative arguments–one to convince someone to use the item, and one to convince someone not to.
There are a couple of different ways you could approach this task; try to do each one at least once. First, be as honest as you possibly can. Force yourself to look at your item from the point of someone whose needs are different from yours and imagine what might make them want or not want to use the item. Second, be as not-quite-honest as you possibly can without ever lying outright–slant the truth, make sneaky omissions, imply things that aren’t true, and so on.