Practice

Play with other players, preferably  better than you. It helps to practice all parts of the game:
The stroke, The focus, and coping with the stress.

Think before you shoot

Decide what to do with every shot before you shoot.
Don't shoot until you are completely sure you will successfully complete it.

The Break Shot

In order to get a the best reaction on the break, hit the top ball with more accuracy.
Hit it solid and accurate with less speed, rather than trying to break powerfully.

Back Swing

It is very important to practice on the back swing.
You should take the cue stick back slowly, That way you can control on your stroke.
Try bringing the cue stick back slowly and follow through with a smooth stroke.
If you bring the cue stick back fast, you can't get a good hit on the cue ball.
Keep your head still at the impact of the cue stick to the cue ball.