Mastering the basketball free throw is no easy task, though there are tips and suggestions that can considerably improve your free throw ability when applied in the course of regular practice. What you'll need to do is to create an efficient, personal routine that involves the proper mental focus, physical stance, and handling of the ball. Once you've developed a short, quick routine that prepares and carries you through the free throw process correctly, you'll want to stick with it and practice it over and over to create a consistent technique, rather than constantly trying a new routine or method and depriving yourself of the ritual familiarity.
Specifically, you'll want the elbow of your shooting arm to be in aligned with your bicep and forearm. Keeping your elbow in line will ensure a straighter trajectory when releasing the ball towards your target. An elbow bowed too far out to the side, or bent inward, will send the ball off too far to the left or the right. Your lead foot, or the foot placed out in front of you when you shoot, should also be aligned with your shooting arm. Your other foot should be slightly behind the lead foot, a shoulder width apart. This foot and elbow alignment will go a long way towards keeping the ball from straying off its intended path. After you've trained yourself to consciously align properly through multiple practices, the alignment will begin to happen quickly and instinctually without much involved thought.
Focusing on one specific spot before throwing the ball gives your eyes, throwing arm and lead foot the ability to coordinate a trajectory, much the same way that a gun scope helps you target a shot properly. For some, this point may be the back of the rim, while for others it may be the backboard square behind the hoop. You'll want to keep your eye focused on this spot, at the exclusion of everything else around you, right on through the actual release of the ball. Keeping your head as still as your gaze when targeting your throw is also important.
Nervously gripping the ball before a free throw is a no-no, and will only disrupt the trajectory of the ball. Rather, you'll want to cradle the ball lightly in your non-shooting hand, and place the middle three fingers of your shooting hand on the ball seams, which allows you to use them as trajectory guides. The palm and thumb of your shooting hand will be acting as supports in this particular positioning. Cock the wrist of your shooting hand back at a 90 degree angle, and release the ball from your fingertips, following through the movement with your fingertips high and pointing at your visual target. Shooting the ball from your fingertips, with the correct amount of wrist "snap," will give the ball the arc and the accuracy it needs to get to its intended destination.