During many of client meetings, one of the most said comments are "We aren't really good in front of the camera, can you tell me what to do?" And that is a totally appropriate thing to say since... 99.5% of the population (if not more) don't do this on a regular basis. Heck, I don't even know what to do in front of the camera since I can't see me!
So I consider the first 15 minutes of every shoot to be a warm up. It's warm up for you to get comfortable in front of the camera and it's also warm up for me to establish comfortable communication. Even though there are a few tricks that can warm people up, everyone is different and every couple I interact is also!
So heres a quick tip, don't worry, relax, and be yourself. Simple concept and hard execution, right? One of the big oppositions to being natural is being too conscious of your own body. I'll tell you if you need to tip your head a bit, shift your body ever so slightly, or move your hand there. Other times, I just tell you sit there, relax, and just interact with your significant other while I go off in a little distance to capture you guys as you are.
Or... I'll tell you to shake your head and make funny faces (he actually did this himself). This shot was taken way later in the shoot and you can tell they are relaxed and willing to do silly things. Maybe next time, I'll try opening with this :)
Looking on pinterest of engagement/wedding photos can also help. It's good visual exercise to see how you would want to compose yourself. Pay attention to the direction of the head and the hands! Many times, those two are the main factors of posture.