To simplify the process of making photos look a bit retro I took the time to create a photoshop action that does all the hard work for you. Although, in order to get a completely original look, you need to tweak some of the settings depending on your picture. After the action is run, it leaves you with multiple layers to allow you to edit anything you like before you save it. This is suitable for photos of any size.
In addition to running this action, and to give your photos some additional character, I would suggest adding a texture to your photo after you are finished. I set my textures to grayscale, then a blending mode of Overlay with a opacity of roughly 40% depending on the photo.
After Action (texture applied)
Another Example (texture applied)
Edit: The action was not functioning properly. It has been fixed.