Consistency Is Key
Proper technique in darts grows out of a comfortable stance, which allows you consistency in your throw. The best dart players throw consistently on all throws.
Take a Stance
Line up the target and dart with your eye. Stay focused on the target, not the dart. Try not to move your body during the throw. Even your shoulder should stay stationary, letting your arm do all the work. Once you have aimed, bring your arm back and accelerate in one smooth move. Don't force the acceleration - let it occur naturally. Don't allow the dart to point down at any point during this movement. You will feel the optimal point to release the dart. Allow your arm to follow through and end with your hand aiming at the target.
The most important point is to find a stance and grip that are comfortable for you. If you're not comfortable, you won't throw well. Practice and experimentation will help you find your best technique.