You can't get rid of that double chin overnight, but what you can do is help camouflage it with the right haircut. There are several hairstyles--in either short, medium or long length--that can help reduce the appearance of your round face.
In order to detract from a double chin with a short hairstyle, choose cuts that incorporate height into the style. While a short hairstyle cannot physically hide a double chin, it can draw the eye away from the chin area by creating interest and volume at the top of the head. The extra height will also make your face look longer, balancing your round face shape.
A medium-length hairstyle can camouflage a double chin as long as it sits at the right length. Avoid any medium-length hairstyle that ends above or at chin level. This will only draw the eye directly to your problem area and make it more pronounced. Instead, choose a hairstyle that sits under your chin or longer.
A bob hairstyle works well at this length. Have your hair cut into either a one-length or layered bob hairstyle that sits past your chin. This will draw the eye downward, away from your jawline. It also creates the illusion of a longer face.
A layered shag hairstyle also works well to hide a double chin. Just don't create too much volume at chin level or your hairstyle will accentuate your chin troubles. Instead, have the layers cut slightly shorter around your face and blow-dry them to sit inward, against your cheeks and chin. This will create the illusion of a narrower face and help physically hide the double chin.
Long hairstyles should follow the same general rules as the mid-length cuts. Use strategic layering around the cheeks and face to help hide your double chin. Also, if possible, avoid having bangs across your forehead or a center part in longer hairstyles--they serve only to box in your face and accentuate the roundness. Instead, choose no bangs or a side-swept bang to show some forehead and make your face look longer, and an off-center part to break up the symmetry of your face.