Who hasn't had a photo that you wanted to print or post, but it wasn't quite the proportion that you needed? Even if you're not a Photoshop guru, Photoshop's Content-Aware Scale feature can help stretch a photo to fit perfectly without disturbing your subject.
Greg Benz demonstrates how to selectively expand the sky or background to fit a specific form factor in this video. It can even be used to shrink an image. The key is how Greg shows how to preserve certain areas of the image. You don't need to be proficient in Photoshop; just follow Benz's simple instructions.
It's not often that I have an image that I need to modify like this, as I prefer to have everything as natural as possible. However, if the need arises to alter a photo for a client, I'll do so. The few times I've done this, it has been when I needed that extra little bit along the edge for a specific crop.
Until I learned this trick, I would sometimes use Content Aware Fill, but I always felt guilty when doing so. I feel like the Content Aware Scale method is similar to using a different lens to bend the image, as it just stretches pixels that are already there instead of making up new ones. I like videos like this because they demonstrate how to do something that you probably couldn't figure out on your own unless you've had a lot of Photoshop experience.