Here's a great drill that'll help you develop game-condition free throw shooting.
Since most free throws happen after a few minutes of running up and down the court in a game, it's good to practice free throws when you are tired. It's really simple to set up this drill. Just sit several extra balls on either side of the free throw line.
Shoot your first 2 free throws, then sprint down to the other end of the court (to the opposite free throw line or baseline) and back again.
Shoot 2 more, and run down again. Keep doing this until you've used all the basketballs you've set up near the free throw line.
You can do a set number of shots and sprints, or you can have a goal of shooting a certain percentage. I recommend doing sets of at least 10 shots and shooting for 80%. If you can't make 8 out of 10, then you have to keep starting a new set until you make the 80%.
Combine this drill with regular free throw practice and you should see an increase in your free throw percentage.