Keep your elbow straight, follow through and hold the follow through. After each shot just note where your arm ends up and how the successful the shot was. Make adjustments according to how good the shot was/wasn't Above all, it just comes down to practice and repetition.

Google 'Dave Hopla free throw shooting'. Apply Dave's techniques and fundamentals, then practice, practice, practice!

Shooting fundamentals and repetition/practice is obviously key.

On the mental side:
* Decide to like free throws.
-- particularly if you've been missing a lot or important free throws. This will make doing the practice easier as well. They're "free" points, everyone loves free points. 2 points and foul on your opponent is better than just 2 points.
* Have a routine and stick to it.
-- In practice and in the game.
-- Your routine isn't just the physical, relax, shut everything else out and concentrate.

All the advice so far is spot-on.

One more thing on the mental side: visualisation
Before you take the free throw, imagine taking the shot and the ball going through the basket.

If none of the above works, there's always Dwight Howard's online correspondence course.
They're doing a two for 1 offer at the moment where you get "David Lee Teaches Defence" for free!

If you are a good shooter from the field but not a great free throw shooter I'm guessing your shot is a little flat?

If that's the case, try to get a little more arc on your jump shot and it should spill over to shooting free throws.

Would need to see your form to give any more helpful advice...

The best thing you can do is not rush. Just take however long time you need. Take 2 or 3 bounces (whatever is more comfortable), steady yourself before shooting. Have a routine, stick to that routine. I like to step away after my first free throw, then set myself up once I get the ball returned to me. This way your aren't anchored to the spot and can re-set for the next.
Then once you have something that feels comfortable, just repeat the same thing over and over. Learn how a good shot feels, learn how a bad shot feels. You should pretty much know if a shot is going in by the feel.
Try make practice as game like as possible. Put pressure on yourself to make shots in practice, that way you have the same emotions going in the game situation.

Thanks for all the advice so far.

Its gotten so bad that I just try to avoid free throws when in game. I pass the ball off before someone fouls me and I don't attack the ring as much as I used to.

Its really just gotten into my head. I play in a competition where stats are recorded and I dread looking at my stats seeing that I shoot 30% from the free throw line or something.

I feel that it definitely has to do with the pressure of shooting them in game and how I know in the back of my mind my percentage is being recorded.

Another thing that sometimes gets overlooked: are you squared up properly to the basket or are your feet at an angle?

Also remember that even the pros who shoot consistently poorly in games often shoot at a much higher clip during practises: which speaks volumes about the mental aspect of free throw shooting. If it's a mental thing and there isn't much wrong with your form, tinkering with it can make things worse.

Are you making them ok at practise?

Bend your knees Katie!

Check the height in your release compared to when your shooting normally? Free throws your not trynna shoot over anybody so most people shoot under the basket or just short of it. Definetly set your feet. Try and shoot down and deep , do you have a good arc?
Think less rotations when your practising by yourself, the ball will more then often just hit the back of the ring and fall in . Bend your damn knees and always follow thru.

take all of this great advise, then fake it till you make it. The next time you go to that line you have to speak nothing but confidence into your own head. You will make this next shot. Great shooters believe that the next one is going in. Also, always remember the ones that went in. The next one will be just like it.

And even then don't shy away from contact. A foul on an oppposition defender, especially an interior one, is a bonus even if you miss both freebies.