Having a solid defence gives your team a bigger margin of error. With a shaky defence, even if you get the other teams flag, you will probably end up in a risky cross-cap situation, and a single mistake from any team member will most likly result in a score for the other team. With a solid defence, your attackers don’t have to worry whether their flag will be there when they get back, and it gives the whole team more confidence.
Certain players make better defenders than others. Defenders ideally should be solid deathmatch players. Defenders will see more combat action during a game than any other position, and they need to be able to quickly take out attackers. A cool head is also a big bonus, as defenders regularly find themselves taking on numerous attackers at a time, and being able to calmly act when the need arises is essential. Being warmed up before you play is always a smart idea before a match. Active CTF servers are ideal, and DM servers are also great to sharpen your combat skills before a game.
One skill that comes with practice, but is vital to being a good defensive player, is being able to cover long distances at speed. If the enemy gets your flag, you need to know the fastest route to intercept them. Good co-ordination and communication with your team-mates will also help you catch the flag carrier far quicker. Knowing how to use the translocator effeciently and knowing the fastest routes to cross each map are the most important things to practice.
There are two general styles of defence, passive and aggressive. What style defensive players use depends not only on their playing style, but also the map. Passive defence involves staying back and letting the enemy come to you. Aggressive defenders actively take the fight to incoming attackers. Some maps require just one kind of defence, while others allow for a far wider scope of defensive tactics.
What weapons you choose to use are very much personal preferance, and can be affected by what style of defence you're using, as well as the map itself. Whatever weapon you choose, the important thing is that it needs to be able to give out a lot of damage in one go. You need to be able to stop a flag carrier (or his escort) dead in their tracks as soon as possible. The longer you take, the nearer to scoring he gets, and the more likley it is that he'll get more backup. Personaly, I find the rocket launcher is a very effective and flexible defensive weapon, but it is less effective in the more open maps.
In a small, tight flagrooms like CTF-Citadel, the bio rifle and flak cannon can be awesomely effective, as is the rocket launcher. In wide, open flag areas like CTF-ElectricFields, weapons like the lightning gun, shock rifle primary fire and the minigun are the most effective. One very useful defensive weapon in most situations is the shock rifle combo fire. It deals out massive ammounts of damage over a wide area, and is ideal for blocking off corridors and firing into groups of attackers near your flag.