Ever play one of those players who can seemingly block everything back? How do they do that? It's almost as if they are playing the video game Pong - they just don't miss. You can do the same thing. Here are three keys.
First, get your racket behind the ball. This may sound simple, but it's key. If you get the racket behind the ball, then all you have to do is have the right racket angle, and the ball goes back! But how do you get the racket behind the ball? That involves proper ready stance, a clear mind, and moving your feet.
A proper ready stance means you are equally ready to move either direction. This usually means the racket tip is pointing directly at the opponent's contact point. (Some players who contact the ball quicker on the backhand hold the racket slightly turned to the backhand, since they have less time to react on that side. If they do, they often compensate by having their feet in a slight forehand stance.)
A clear mind means you are ready to react to the ball rather than trying to anticipate. Just watch the opponent as he hits the ball and react. Don't think or guess; just react. The only time you anticipate is when you have popped the ball up so weakly that guessing where the ball is going the only way you can react to the next shot. However, if you can read where your opponent is going early in his stroke, that's not anticipation - that's reacting, and you should react. Never react so early as to let the opponent see this early enough that he can change directions.
Moving your feet means exactly that - step to the ball rather than just reach. Some do get away with mostly reaching, but that limits your range and hurts your control.
Second, good contact. This means blocking firmly against heavy topspin. If you just hold the racket out, the spin will take on your racket and shoot up. So put a bit of firmness into the shot. Learn to use the same contact every time so you can develop your timing and precision - and then learn to change speeds.
Third, practice. Lots and lots of practice. There are plenty of players looking to work on their attack, so oblige them. The more you practice your blocking the more you'll become one of those players who seemingly block everything back.