Put Yourself in Their Shoes
My first suggestion would be to consider your opponent and how he approaches the match. Is he an aggressive blocker, looking to use his block to out-maneuver you and set up his own attack? Or is he the wall-like defender, content to simply put every ball back on the table and wait for you to beat yourself as you hit harder and harder, and try to angle the ball wider and wider?Aggressive blockers must be handled with care, since any lapses in concentration on your part will be swiftly punished with lightning counter-attacks. You can't afford to let up against these guys at all - you must stick to your game plan at all costs. Every weak attack or bad push that you make will cost you. I'll talk a bit more about what your plan should be in a little while, so bear with me.
Against passive blockers you have a little more margin for error, since you know that you can push the occasional high ball, or make a less than perfect attack, and they won't punish you for it. The mistake to avoid here is getting sloppy and losing your concentration or form. The passive blocker is still out to win, make no mistake about that. It's just that he would rather wear you down bit by bit, letting you beat your brains out against his defense while he reduces you to a long dribble of sweat (a line I stole from squash great Jonah Barrington).
Passive blockers love nothing more than watching you slowly lose your cool, your temper, and your mind as they stick their bat out and plod the ball back again and again. So remember, you have a little bit more margin for error, not a lot. Use it intelligently and to your advantage, by playing with a little more spin and less pace while you wait for the right ball to put away.
Get Rhythm (Not!)
The typical blocker thrives on matches where he can play at his own rhythm. It's up to you to find out what that rhythm is, and avoid it most of the time. Does he prefer to block against flat, fast drives? Or is he better against the slower, spinnier loops? Does he like to play crosscourt or down the lines? Does he favor his forehand or backhand block? These are the things you need to know in advance, or find out fast when the match starts.Remember, the pace of the ball, the spin on the ball, the depth and placement of the shot, and the height of the ball can all be varied by you - somewhere in that combination will be a return your opponent is not comfortable against - probably more than one. Your job is to find out which ones they are.
It's OK if your opponent hits a few winners while you are trying to find his weaknesses. It's the price you pay for not scouting him out in advance. Anyway, knowing what he likes helps you to narrow down what he doesn't like. Once you find out what that is make sure you give him lots of it.
Patience, Young Grasshopper
Most people would advise you to be patient against blockers. This is not bad advice, but it is important to add a proviso - don't try to beat the blocker at his own game. Just like the old adage goes "Don't try to argue with a fool - they'll drag you down to their level and beat you with experience", don't suddenly try to out-block the blocker - you'll be playing the game that they have been practicing for years.In this case, patience means waiting for the right ball to attack with power, and controlling your aggression until it comes along. Against aggressive blockers you need to use whatever return they don't like often, while you wait for that set up ball to come along. Whether it's slow, heavy spin deep on the table, or light high topspins to the wings, grit your teeth and keep using it until you get the return that you know you can put away with power. Against passive blockers you have a little bit more leeway, and can push a bit higher, and take less risks with your normal attack, since you know that they aren't likely to counter-attack you too much.
Some folks take the opposite approach, trying to blast through a blocker using their hardest attacks all the time. This is known as the 'I'm going to play my game regardless' approach, and can sometimes even be seen at world class level (Kim Taek Soo - I'm talking to you!) While this is a tactic, this will usually only work if your level of attacking is well above your opponent's level of blocking. And if that is the case, you could probably beat him half a dozen different ways anyway. It's when you come up against an opponent near your level that you can't just hit through that good tactics become important.