Dressing cool is a matter of studying what is in style and having natural fashion sense. To dress cool, you need to define your personal style and have something unique that is solely yours.
You can find out what is in style at the moment by studying fashion magazines, runway shows and by viewing what celebrities are wearing. You can also find inspiration from old advertisements, movies and people whose style you admire. When picking items that are in style, it's best to choose one or two trends to sport at a time. This prevents you from looking overdone or gaudy.
Dressing cool also involves finding your personal look. This look is one that you look good in and can pull together well. It needs to be a style that fits your personality; otherwise, you potentially look out of place. Once you've envisioned your look, perfect it. Make your look unique by having a signature piece. Your signature piece should be something that only you wear, if at all possible. This could include a vintage accessory, a hat or a certain type of clothing. Find unique items at thrift stores or flea markets to make yourself stand out from the crowd.