One of the most common mistakes in portraits is not checking your background.
I know, when photographing people, there are so many things going on, often we get so occupied with the person(s) we're trying to photograph, we forget to check the background.
Once you are about to press that shutter, take a second to check the background. Is there a tree sticking out of the head of one of the people in the photo? A fence "decapitating" the subject? Something distracting in the background that takes away from your portrait? If so, rearrange. Go lower, higher, to the side.
Avoid tunnel vision. It's a rookie mistake, easy to do... but even easier to correct. You get so excited about everyone looking good that you don't see until you pull the pics up on your computer that there's a giant branch that seems to be poking your subject in the head. Not the effect you were hoping for.
So take a moment and make sure that the area around the subjects head is clean, you'll be happy you did.