The primary goal of the marker should be to prevent a break the mark pass, not to obtain a handblock. Of course, a block is terrific and has potentially devastating psychological effects, but usually you lose more by trying for the block all the time. On the higher levels, going for the block too much will open up the other throws for upfield passes to receivers who will not be marked. On the less skilled levels, overplaying for the block will enable the thrower to get off a short complete pass instead of forcing up a low percentage pass.
The key to marking is to keep your balance. Most people can't extend a lot without getting completely off balance. Bend at the knees, so the weight is on the balls of your feet, and your upper body is directly over your feet. You want to be able to step left or right along with the thrower without having to shift your weight a lot. In any event, you should be able to return to your original position without having to move your other foot. If you are coordinated enough to block with your feet, more power to you, but I have seen only a handful of footblocks in the last few years (and one of them happened because the marker was completely off balance because of a fake but still managed to throw a foot up blindly in the spot where the thrower was throwing). There was a thread a while back about the dangers of footblocking, also. Anyway, practice shuffling your feet back and forth as the thrower pivots, also keeping your weight over your feet, your upper body upright, and your hands low. Except for the high backhand and the scoober, most throws go underneath the marker's arms (when they almost come in contact). Until a thrower has beaten you with a high throw, keep your weight low.
Another thing to help your marking is to learn individual thrower's habits, such as noticing a penchant for the high backhand. We all have our rivals that we play frequently, so notice the types of throws and fakes the players you cover use. If they regularly throw over the marker, be ready to take a step back to make it more difficult. If they often step into the marker, be aware of that. If they always throw the low inside out forehand, be ready for that. There is no one perfect position to set up in. On some people you should set up as tightly as you are allowed, on others, you want to back up a little.
I'm not going to go into the ethics of fouling, but you probably don't want to adopt the slogan, "Foul early and often". It's usually not worth your while to make an obvious foul to prevent a throw. Often, the thrower will overextend himself trying to get around you, and his throws won't be as good. If you systematically foul him in those situations, you are giving him a free throw or a new count. Your goal should be that if the thrower is going to throw against the mark, either he will have to travel or get himself way off balance in order to do so.
If you are statistically minded, you might want to keep track of a few things. Take numbers on how many throwaways happen for each marker, and how many times their marks get broken. It could become an article of pride.
Offensively, you want to remain calm when faced with an aggressive marker. Keep the marker off your pivot foot. If you are fouled, make sure you get a throw off. Don't let them get away with hacking you and getting a reset. Consider where you want to throw the pass, and what you have to do to make the throw easy. The marker usually can't see downfield, so they can be susceptible to fakes. Don't pivot and fake wildly just for the sake of doing it. For me, I have a couple fakes that are almost part of my throwing motion (and if you watched me enough on tape you could figure them out (not that you could block them)), but nothing else. Almost everything that looks like a fake is an attempt to throw the disc but the throw isn't there, so I pull it back and try again. Anyway, when you practice your throws, practice them as they would be thrown in a game, either off a pivot or off a fake. In your team drills, incorporate a marker into some of them, even if the marker just stands on one side of the thrower. And if you're fouled, call it calmly. Don't scream like you're being attacked, just say, "Foul" and throw the disc.
Marking and throwing against a mark are all things you can work on with as few as three people, and have tremendous payoffs. At least in the club open division, one of the most significant differences between good and bad teams is the number of players on each team who can beat the mark successfully. That, and their ability to spike.