There are a number of common ones, including:

LEANING over the line to get closer to the board. This one is a real loser, since 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 place most 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. 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.

LUNGING or lifting the back foot off the floor during the toss to get a harder throw. Lunging is one of the worst habits, as it affects the entire body and throw. Lifting the foot even partway from the floor deprives the body of good balance during the crucial moment of follow-through. The strength required to reach the board with any normal dart is minimal, and for best accuracy should be provided only by the fingers, wrist, and forearm. Missing the board or hitting too low often cause beginners to think that more power is needed. This is rarely true, as one can tell by the fact that the missed darts usually stick in the wall, which is quite a bit harder than a bristle dartboard. The problem lies with the accuracy of the throw and follow through. Even small children can be taught to throw accurately without lunging or using the shoulders in a throw.

HOLDING 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 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 point-up, 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 darts also may stick in the board at odd angles, especially after a long period of play when concentration starts to slip a little.

THROWING THE DARTS, 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 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 point is too short and should be changed at a darts shop.

SPINNING the dart as you release it to add stability. WRONG! This is wasted effort at best, and can actually make your game worse by causing uneven release. Most darts flights are not shaped to properly induce spin, and the darts actually fly for too short a distance (about 5 feet) for aerodynamic spin to be a stabilizing factor anyway.

Spinning the dart is often done inadvertently, and is a symptom of uneven release. As the hand opens, if the thumb leaves the dart before the fingers, the dart will roll sideways off the fingers, causing the dart to spin. 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.

The above borrowed from CyberDarts
Copyright 1997 R.C. Osgood.