Nothing can ruin a photo more than your subject looking uncomfortable. As the photographer your first job before everything else is making your subject, or subjects feel comfortable.
Relationship
What is the easiest and fastest way to make your subject, or subjects comfortable? Relationship. Be yourself, be social, let them know that you enjoy being there, taking photos and capturing their story.
Smile be genuinely happy that you are out there capturing photos. You don't have to be ear to ear grinning the whole time but you should be joyful and focused.
Be Professional Photography is a profession. There is a lot of people out there with cameras who say they are photographers but they forget that photography is a profession. This means that there needs to be intricacy and dedication with your craft.
Respect their time This is the hardest for me, because every photo I take I think I can do the next one better. Trust yourself, and trust that your eye will carry you. I recommend a shot list. A simple list of the different images you discuss with your subject (s) helps them know what to expect and gives them a schedule and understanding that you know what you are doing.
Be Yourself
You are you, nobody else is going to take the photos you take, because even if you have two photographers standing next to one another and taking a picture of the same thing, those images will be different. Be confident that the skills and the time and money that you have put into this craft will not go in vein. Keep focused, smile, be professional and respect your subjects time. Get out there and create!