A steady Stance is very important. Don't lean way over the line to get closer to the board. This one is a tough habit for some people to beat, but try.. as leaning robs the darter of stability. The feet and legs should be positioned in a solid, comfortable, and relaxed stance, with weight distributed to both feet. Excessive leaning places nearly all of the body weight on one foot, tiring the shooter in long matches and damaging accuracy in the short run.

The few inches gained by leaning over the line are simply not worth the huge loss of balance and stability. Plus, leaning lowers the shoulder, forcing one to throw upwards, fighting gravity. Leaning also usually means tensing the major muscles of the body to preserve balance. This often results in a jerky release and poor follow-through, since the body is already off-balance.

A number of long-time players report back, knee, ankle, and foot pain, from spending many years standing on one foot while playing darts. Even in the short run, leaning to throw will cause minor pain in the small of the back. Especially for older players, a firm stance will stop this discomfort, both while playing and the next morning!

Think about it..

In what other sport would you drink a few beers, try to stand balanced "tiptoe" on one foot, and then try to consistently hit a small target with a sharp pointed object?

In EVERY competitive sport, Accuracy begins with a Solid Stance!
The dart should be held in a level position, and maintain as level a stroke as possible. Don't hold the dart sideways, or in any other position than level and pointed at the board. Skill at darts, or any other target sport, means being able to perform the same motion exactly the same way, time after time.

Common sense, as well of years of studies in other sports, show that all non-essential motion should be avoided and discarded from the darter's routine.

In Darts, this means that if the dart is to strike the board at a level attitude (nearly always the best), it should be held and thrown from a position as close to level as is possible.

Any other position (such as dart point-up, dart point-down, or sideways) means extra motion of all the hand and wrist muscles to correct the initial starting position. Pure wasted effort... and usually futile, since the dart will likely leave the hand at an angle and wobble all the way to the board.

The correcting motion needed to get the dart pointed back at the dartbaord imparts inertia to the dart's mass, away from the direction of the target. Then the darts will often stick in the board at odd directions. After a long period of play, when concentration starts to slip a little, this can really be obvious, with darts hitting at all sorts of different angles.

Instead of "throwing" the darts, instead just gently "place" them in the dartboard with a smooth motion of your hand & fingers. Throwing like a baseball is unnecessary and even dangerous, as a dart thrown too hard may hit a wire or other object and bounce clear across the room to hit someone. Dartboard wires get bent and the bristles crushed from this type of abuse.

Fortunately, "baseball throwers" usually stop after a while, either due to the laughter of spectators or the frowns of the darts bar manager. This method is also hopelessly inaccurate, as all of the major strength muscles and very few of the fine control muscles are used. A dart should never be thrown so hard that the front of the dart barrel touches the bristles. If this happens when a dart is thrown normally, then the dart's point is too short and should be changed at a darts shop.

Avoid Spinning the dart as you release it to add stability. Spinning the dart is often done inadvertently, and is a symptom of uneven release, usually a side effect of wrapping fingers OVER the dart, which then forces the dart to roll off the fingers on release. Instead, grip a dart gently from the sides, so that it easily flies free upon opening the fingers.

All parts of the hand should leave the dart at nearly the same time to ensure level flight. To achieve this, make opening the hand a positive motion, and open the fingers and thumb rapidly to an extended position, ending up pointing at the target. This will also help keep the flights from touching the fingers as the dart leaves the hand.

As an exercise to get that qucik release, imagine dipping your hand in a bucket of paint, then fling it at a spot on the wall. If your hand ends up towards the floor or ceiling, that is where the paint would have gone! Let your fingers end up naturally open and pointing at the exact spot where you want the dart to go.

Purposely adding spin to a dart-throw is wasted effort at best, and spinning the darts can actually make your game worse by causing uneven release. Most darts flights are not shaped to properly induce spin, and the darts fly too short a distance (less than 6 feet) for aerodynamic spin to be a stabilizing factor anyway.
Taken from article at Cyber/Darts