There are a few must in a good shot.
1.Sighting --- How well you see the angle, always walk into the object
ball. Do not approach the object ball by walking around the table. 
What you should do is, walk to the object ball, see the angle while
standing, use the time to chalk. When you know what angle, walk into the
shot, lay your hand down while keeping your eyes on the object ball. Bend
down and aim, your eyes should be looking at the object ball all this time.

2.Stance --- A good stance is right leg straight, left leg bent, left foot
pointing almost at the object ball, right foot about 30 degrees. Rest your
body weight on your right hip.

3.Bridge --- A good bridge is important to prevent curling of the cue ball.
Never poke into the ball unless you want to masse. Strike along the ball,
cue should be almost parallel to the table surface. Same applies to screw,
stun, follow and plain-ball. Your palm should rest on the table for normal
shots, unless spider or cushion. For spider, your bridge-arm should be
straightened,cue should be guided under your chin as per normal, not by the
side of your face.
 
4.Timing --- Unless you are trying to hit a push-shot, ie. one that is very
slow, good timing is required. A good rhythm to follow, used by JOHN
HIGGIN, is to:
AIM...AIM...aim aim...pull back......and strike.
Note: "AIM" means large movement of cue-arm, "aim aim" means small movement
of cue-arm.

You will notice that with good timing, you can screw with a lot of power. I
have tried before, cue-ball 2 feet from brown...angled about 20 degrees,
after potting brown, the cue ball can screw all the way to around the pink
spot. Remember to followthrough.

5.Stay down --- a psycological factor about snooker. When you stay down and
watch the object ball roll into the pot, you will tend to take that shot
more seriously and aimed better. One common mistake I noticed among many
half-past-six players, they bounce up immediately to see the shot after
they strike.

6.Strength control and cue-ball knowledge. The biggest problem with most 
players is their strength control. Their cue-ball runs all over, causing 
them to have to hit long ball and cushion ball.
One advice, only use power when you you need to screw alot. I have seen
alot of players who like to "Tia Sia (hear sound)' when the shot is quite
easy, but they end up having no placing. You should observe every shot and
note how the cue-ball run after the shot. Generally, a 90 degrees guide if
you poke center, less than 90 if you poke top, more than 90 if you poke
bottom.

7.Placing --- A game usually starts with a long pot, try to bring the
cue-ball back to around the blue spot. Pot blue and bring it down to where
all the reds are. From there, you can start to plan how you want to combo
with the pink and black. Yellow, brown and green are usually not played,
unless you really run out of position. Hitting any one of these, you should
try to bring the cue-ball back to where all the reds are, using either
screw (need alot) or using follow-ball.