One of the most important keys is rhythm. You want to develop the same rhythm or routine every time you're at the line. Don't think just DO.
Everyone has their own routine. Here's mine:
take in a nice looooong deep breath and exhale
bounce the ball once, then twice
bend your knees
then release the shot
Some key things to remember:
keep shoulders square, facing the basket
feet spread shoulder width apart
right foot in front of left foot (for right handers)
use your legs, bend your knees and spring upwards as you shoot
Sometimes, people will try to psyche you out or distract you, just remember how perfect you were in practice. Oh and it doesn't hurt to clear your mind and block everything else out. When your mind gets to loud, that's when you miss.
Remember, this is a free throw; it's FREE. With good technique and practice, there's no reason whatsoever that you shouldn't be automatic from the line.
Here's a video of Michael Jordan offering some valuable practice tips and insight on free throw shooting: