#1 - Make them laugh!
It's very important for the couple you are photographing to be completely comfortable with you. So what I do, is make jokes. I make a fool out of myself. Get to know the couple, talk to them before you start shooting. If the couple is comfortable with you, most likely, you are going to have great, not tense pictures. The candid moments. Some couples are going to be like models, but some (most) won't. They will have forced smiles and forced poses. Making them laugh (no matter what you do) will allow them to truly smile.

#2 - Get in Close
Most pictures of couples that I like are of close ups. The close up of their hands or their eyes. Really closing in on a moment that no one would have focused on, if you didn't ZOOM in on it. Small things always count. His hands on her leg, her feet curling up, etc. Find those moments and close in on them :)
This photo is taken from http://pascallphotography.wordpress.com/

#3 - Know the Couple
I mentioned this a little in the first point, but it's very important to know the couple. When you know the couple you can then take photographs that represent them. Ask questions about how they met, what they like to do, etc. Get a feel for who they are and then take photographs that represent that.

