The key to hanging posters like a grown-up is to keep your posters to a minimum. Unlike your teenage bedroom that may have looked like you had posters as wallpaper, posters shouldn't take up all your wall space.
On the other hand, when done well, you can cluster posters in a room or single wall to create a mosaic effect. Use different sizes of posters for best results. Another option is if you have a room dedicated to collectibles or a hobby, hanging multiple related posters around the room works well.