This is why shooting free throws is such an important part of your game and many just don't practice enough. Lots of times the win-loss result of a game is because of free throws.
One thing that helps someone is repeating the same behavior pattern before you shoot. A regular task so to speak that will help your concentration and focus. A player might dribble the ball three times before he shoots a free throw and not doing so would completely throw his focus out of wack. One might lift his hands in the air a few times before he shoots it. One might touch his cheek a few times. Look at mostly all the great free throw shooters and they all have a ritual before they shoot. It could even be a saying to yourself, even a prayer. Doing this will bring familiarity each time your at the charity strike and will really help alot.
Another thing to do is to visualize in your mind what you are going to do before you do it. When you see the basket going "Swish" through that net in your mind there is more of a chance that it will happen.
Also trust in yourself, this kind of goes back to the visualization but you must have self confidence. If you look at a player like Shaq O'Neal he has completely lost confidence and just knows that he will never be a 80% free throw shooter. He has worked with the best trainers and still can not shot free throws - Don't ever let that happen to you, there is always room for improvement. Don't delay and hesitate when you shoot, this will not help at all. Just do it! You just seen yourself swishing it so do it!
Confidence is a big thing with free throws. When I made the 690 lots of times I would miss one free throw, get down on myself and visualized I would miss the next one, guess what, I missed the next one. Sometimes I would miss 5 in a row because of the lack of confidence, that it why I was a 69% shooter. At times I would hit 20 in a row and then miss 5 in a row. Do that and your only a 75% free throw shooter!