Raise Your Head
This is one of the best ways. Act as if you are looking at something that is at a height. This will make your chin look less fat as it will be distributed and you will be able to have a picture of yours taken without having to show your double chin.
More light can also be useful in hiding the double chin. All you need to do is to push your face forward a little and have a good bit of light while the picture is being taken. Although make sure that the light is not too strong and over powering as it will over expose the picture.
If it is winter, this is a good idea as well. Wrap a muffler around your neck as you are getting your snap taken. Make sure that it does not look out of place and will be ideal for group shots. It may actually look a little trendy too and you may standout in the group of people because of it. Ladies can use a scarf for the same purpose.
Rest it on Your Hand
This is another technique that works well. While the picture is being taken, pose for it and place your hand underneath your chin in an artistic manner. This will help you not only to look great and stylish but will also hide the chin perfectly well.
Another way of taking the double chin out of the equation is to take the picture from an angle from the side rather than the front. Having the position of the camera a couple inches above your eye level will help even more and should provide ample cover to the chin.