It should also be kept in mind that ultimate is a team game. Just because you have shut your man down doesn't mean that your team won't be scored on. Part of playing good defence is playing good Team Defence.
By short here, we mean short as in close to the disc, not vertically challenged!
Generally the best spot to set up is about 3-4m in front of the receiver, slightly towards the open side. The defender is relying on the marker to stop any throws on the break side, so he allows a small amount of leeway on that side. It is much more important to make sure the receiver does not get free on the open side. The receiver will usually run at the defender, fake to throw the defender off, and then run hard in a direction. To avoid the receiver getting free, the defender needs to be able to stay in front of the receiver, usually by running backwards so that he can see the receiver. As soon as the defender has to turn around it is much easier for the receiver to get free. In brief :
Marking deep players is slightly different. To mark the deepest player, the defender should usually stand just behind the receiver, and on the open side. This is so that if the thrower hucks long, the defender is in a better position to get the disc, as he is further back. Obviously, it allows the receiver to make easier cuts in for the disc, but this is much better than having a goal scored.
Once the disc is called up, the defenders should glance around to see where the disc is. It is possible that they could make a play on it. This is more important if the call is "Up long", as conceivably quite a few players will have a play on the disc. The defenders need as much time as possible to have a chance to establish good position early.
If used correctly, switching is a good method of shutting down a good cut. If used poorly, it is a good way of leaving a man completely open. Switching requires good timing and understanding. There are two main sorts of switch.
The first is the lazy switch, where the players are simply switching because the other's receiver is closer. As with all switches, both defenders need to know who their new receiver is. They should also make sure that they are not causing a defensive mismatch eg a short fast player marking a tall player.
The second is a switch on to a cutter. The player calling the switch will usually be near the front of the stack and will see a cutter coming from deep. He should wait as long as possible to call the switch, and at the last minute change receivers. This allows the other defender time to pick up the free man, without giving the free man too much time to plan a cut. The switcher needs to talk to the other defender to make sure the free player is picked up.
Once players have played together for a while, they may be able to switch just by glancing at each other and nodding, or something similar. Switching is not recommended for beginners, as it teaches bad habits.
Poaching is generally looked down upon in ultimate as "a bad thing", mostly because it is a sign of laziness in defence. If used wisely and sparingly it can be remarkably effective, but it leaves free men around the field and can result in large yardage gains for the offensive team if punished. There are a number of different ways of poaching.
The first is a straight out zone poach, where the poacher takes up a position in the main cutting lane and basically ignores his own receiver. This is only effective when either the thrower is weak or the receiver is weak, so that the chance of a pass from one to the other is small.
The second is a temporary switch, where the poacher sees a cutter coming from deep and switches for long enough to discourage the thrower from throwing to the cutter, before switching back to his own receiver. The switch can involve actual marking, or just a flaring out into the cutting lane but has the same effect.
A third type of poach is an active attempt to get a block. This involves the poacher seeing somebody else's receiver in a position to receive a pass, but where the pass if thrown could still be intercepted. The poacher hopes that by not watching the disc, the thrower will think the pass is on, and hence throw it. The poacher has to guess when the pass will be thrown, and then turn around hoping the disc will be there to intercept. This can be effective if the timing is right.