I distinctly remember taking a photo with my siblings when I was 11 years old. My sister was one at the time so the photographer had to resort to using bubbles and squeaky toys to get her attention and snap the photo. Obviously that method wouldn't work on a couple in love...right?
Getting a couple to remain calm and candid during a photoshoot is one of the toughest things. Children are easy because they're naturally candid. I've read on some forums of a couple methods that can help you: say you're changing your memory cards, say you're cleaning your lens, etc - anything to snap them out of being self-conscious and remaining stiff.
One method I'm sticking by is using laughter as a motivator. I poke fun of the situation at hand, my funny t-shirts, a comment about a television show the couple watches. Something that will catch them of guard or make them laugh. Sometimes I don't even need a big belly laugh, even a little grin will do. It's natural and makes for great interaction.
So when you book me for a photoshoot, don't be surprised if I start making corny jokes. If that doesn't work, I'm pulling out the bubble wand.