Know your job. Blockers know their primary blocking responsibilities. A middle blocker's primary job may be to block the quick attack, then try to block any other attack. An outside hitter's primary responsibility may be to block the live setter, then block any other attack. Stand at the net.
Blockers never move towards to net to block.
This may give you an extra inch of vertical, but will cause more net violations and sprained ankles. When the opponent has the ball, blockers stand right next to the net. You need to learn to feel comfortable being very close to the net. The reason is that it takes more timing to bring your hands up from your waist, and there is also a risk that you will catch your hand on the bottom of the net. If the live setter is really far from the net, then you know that he won't attack.