One of the most common 'film era' techniques is Vaseline on the lens. It creates a beautiful dreamy look to the photo, and the beauty is that it can be wiped off easily.
Modern lenses have special coatings on them to prevent glare etc, and there's a chance that Vaseline (or other petroleum jelly brands) could damage it. Bad times.
An old uv filter on the front of the lens is a better option. Just screw it on, and smear Vaseline to your hearts content. If it damages it, bin it.