Throwing a dart is not unlike hitting a baseball or golf ball… The key is repeatability. The best way to achieve repeatability is to reduce the number of variable factors to as few as possible. The fewer moving parts you have, the fewer things can go wrong.
Keep your body quiet, no knee bends or bounces. Ideally, the only things moving will be your elbow, wrist and fingers. Mind your follow-through, as a general rule wherever your index finger is pointing in your follow-through is where the dart will land.
Don’t snap your wrist on release, this can create a lot of random motion. Try not to loft the dart to the board, as this introduces another up/down variable to the flight of the dart. A dart that is canted in the board from a lofted throw can block your next target. The best trajectory is as near flat as possible.
Lean as far forward as you can comfortably to reduce the distance to the board. Don’t fall out of your follow-through.
There are any number of ways to address the throw. It comes down to what will be most repeatable. Personally, I move left and right on the line to line up with the particular target. I base this on practice to find the spot that lines me up with the 20, then adjust for whichever number I’m going for. I hold the dart in front of my right eye and focus on the board. This causes the dart to appear doubled in my sight. I then use this to center in my target. All things being equal, I’m then ready to draw and release.
The draw should be smooth and slow, no need to fling the arm back like you’re throwing a javelin. Once you’ve reached the back of the draw, come forward, again smoothly, and release without spinning the dart. Pay attention to the flight of the dart. If the tail drags, or drags and then flips up, try adjusting your release point. If that doesn’t work, you might want to adjust the length of your shafts or the size of your flights for better aerodynamic balance.
If you’re having trouble hitting double-20 or 18, for example, resist the temptation to loft the dart up there. Most times simply raising your arm a bit will allow you to get there while maintaining a flat trajectory.
Most of all, have fun!!