Basic Marking
Marking is an active process, not a passive one. It involves reacting to whatever the thrower is doing to try to make any throw a more difficult one. The harder the marker works, and the better the mark, the less work defenders have to do to shut down their receivers.
So how do you put a good mark on a thrower? The following are general tips to improve your marking if you are not already doing them :
Keep on the balls of your feet. You have to be able to react and move quickly, and this is not possible if you are back on your heels. You have much better balance if you are leaning slightly forwards.
Keep your weight low. Crouch down, or at least bend your knees slightly. If you are marking well you should feel the effort in your quads. This allows you to get your hands low and also gives you much better balance.
Keep your hands low and arms spread. This also helps your balance. Obviously though, if you are trying to prevent a high throw, that arm will have to be up. In general, the arm on the force side should always be as low as possible, since it is much harder to throw a good throw from higher up.
Learn to read fakes. Many throwers make weak fakes without ever intending to throw. If you can pick these up, you will not be sucked in to following them and leaving an easy throw open.
Don’t over commit on the open side. It is not your job to block an open side throw. That is the defenders’ responsibility. If you let the thrower break because you tried too hard to block an open side throw, that is your fault.
Call “Up!” loudly when the thrower throws. This allows the defenders to glance around to see where the disc is, since they will otherwise be watching the receivers. If the throw is hucked, call “Up long”, to allow the people marking the opposing deep players time to see the disc.

Advanced Marking
All of the above tips are generalisations. Most of the tips will apply to most players, but for really good marking, nothing beats knowing the thrower. If you know which throws a thrower prefers, which fakes they use and how they like to break the force, you can modify your marking style and you are far more likely both to get hand blocks and mark more effectively.

There are also times in the game when a standard mark is inappropriate, and something different is called for. Being able to recognise these situations is important, and knowing how to react to them even more so.

Marking off
The marker is allowed to stand one disc width from the thrower, but it is not always best to stand this close. Against strong throwers who enjoy throwing inside-out passes, it is often best to drop back a metre or two. This makes it easier for the thrower to throw wide throws, but pretty much cuts out the inside throws. It also makes the likelihood of a layout block greater. On the down side, it is a little easier to break the force with a wide throw.

Hands low/high?
Usually it is better to keep both your hands low, since low throws tend to be more dangerous, but this is not always the case. If you are marking a player who likes hammers or high backhands then it may be wise to keep your right hand up. This gives you a chance of blocking either of these high release throws

Watch fakes
As mentioned above, try not to fall for obvious or weak fakes. This is easier if you know the thrower, since they tend to use the same fakes to try to throw you off.

Straight up force
There are a few times in a game when a force to one side is inappropriate. Usually this is just after a huck, when you may be marking the person who has just caught the huck, and there is a man free in the endzone. The thrower will be trying to hit this man before the rest of the defence catches up. In this case, it is your job to make it as difficult for the thrower for as long as possible. The best way is to use a straight up force. This means that instead of standing to one side of the thrower, you stand directly in front and try and block any throw. Usually it is best to drop back a metre or two to maximise the time you have to see the disc coming. This type of force will only be used for a few seconds, until the defence has time to regroup.

Preventing the huck
There are also times in the game when it is obvious that the thrower wants to huck the disc, mostly when someone is running free long. You need to recognise these situations and make it as difficult as possible for the thrower to get a good long throw off. This will most often involve a straight up force. Follow all of the fakes, because it is important to make the huck as difficult as possible, even at the cost of an easy short throw. Recognise that most players prefer to huck backhand, so overplay the backhand side of the force to stop them. As with a straight up force, you will only usually need to mark like this for a few seconds.

Where are the receivers?
If you have particularly good game sense or peripheral vision, you may be able to tell where the potential receivers are. In this case you can overplay that side of the force a little. One way you may be able to tell is by watching the thrower’s eyes - it is difficult to fake effectively with your eyes. Another way is from communication from defenders and the sideline. “No break!” is a call to overplay the break side. “Strike!” is a call from a defender to switch the force to the other side for a second, as a receiver is open on the open side.

Above all, remember that marking is an active process, and not a passive one. You are trying to prevent the thrower from getting an easy pass away - make it hard for him. Keep moving, keep your hands active, but don’t let them break.