When persuading people, you don't need to show both sides, you can simply present your own views. For instance, imagine you had to persuade people to buy a certain brand of toothpaste.
You could write about the great taste, or how clean your teeth feel or how white they have become. You could select any aspect of the product to write about as long as you focus on getting them to buy it.
So the first thing is:
Be confident - if you have been asked to write about how great chips are, be sure you have first convinced yourself how good they are. They might even be a healthy option, because lots of doctors are bound to like them and chips wouldn't be so popular if they weren't really great. Would they? You can be pushy here if you are trying to promote something.
Be positive - it's always better to tell how good your idea is rather than how bad other people's ideas are. Write about the things that show your ideas in the best light. For instance, aren't chips just great for a quick snack? Why were chips voted the most popular option in your school at lunchtime?
Be convincing - if you don't seem too sure, or if you can't make your ideas stand out, then you aren't really persuading as much as you could. Remember to be definite and clearly state your point.