Prevention in the form of interrogation and intelligence-gathering is essential; although they are not popular, "enhanced interrogation techniques" on enemy combatants have led to the prevention of many attacks since 9/11. The important thing is to balance what we are allowed to do with what must be done for public safety; this is, of course, a vastly controversial subject.
The biggest impediment, as other editors have noted, is the way in which the terrorists in many ways have the advantage because they can choose any target and any method of attacking it. This means that those trying to prevent such attacks are at a distinct disadvantage, as how can you prepare for something when by definition you are not going to now what that thing is?
Such plans are heavily dependent on intelligence gathering capabilities. Typically they center around denying logistical support to the insurgents, winning support of the local population (hearts and minds) and targeting the leadership of the organization. In the days of Vietnam, counterinsurgency was closely tied to "body counts" in a war of attrition. Modern counterinsurgency is much more targeted and selective in how resources are applied.
Ideally, the participants who need to be included are the relevant members of all the relevant law enforcement agencies, who need to share as much information as possible and analyze it as thoroughly as possible. Failure to do both of these things was part of the reason that the attacks of September 11 took place.
My biggest concern with all of the planning that goes on with regard to counter-terrorism is how the U.S. can possibly be thorough with a country this size. So many things are happening at one time, just in a city. We can be paranoid and think the guy with the knit cap, sunglasses and fatigues is a terrorist, but he may be just another guy—someone's brother or father or son. There seem to be too many gaps: ways people can sneak through the cracks and avoid detection. We have no way of really knowing how many attempts are made, how many terrorist cells are wiped out or how many botched attempts may account for a number of "incidents" that are never reported to the public. And that's fine, I guess, for what would knowing do?
I just cannot conceptualize how we can cover so much ground and so many people. Terrorists don't wear signs saying, "Here I am." When people start carrying on about how many people leave Mexico and come into the US in California, etc., how can we possibly imagine we'll ever get a handle on terrorism as things stand? I don't have the answers, but I believe much more money and man-power would have to be in place throughout the nation to really tighten things up. I don't mean to be cynical. It simply seems a question of challenges vs. assets (personnel, money, etc.).
A huge drawback in counter-terrorism is that these terrorist cells are now largely independent. So, even if we have information on one group, we might not know what is going on in another. This also bring up the point that getting reliable knowledge is difficult. Moreover, the coordination of this knowledge is a challenge as well. Finally, I should mention that America is a big place. Hence, to patrol the borders is not an easy task.
Counter-terrorism involves planning at a number of different levels and in a number of different areas. State and local law enforcement are involved, as are national law enforcement. Intelligence organizations gather information from a myriad of different sources, and even the diplomatic corps is involved in arranging for extradition, freezing assets, and other measures. And obviously, the military, particularly special forces, are heavily involved in attacking terrorist organizations in war zones. Coordinating the vast array of elements involved in counter-terrorism is a major challenge.
The way a counter-terrorism plan is developed varies greatly on who is developing it and who the plan is for. Individual cities and even companies can develop plans, but their resources will be limited compared to the Department of Homeland Security. In recent years, since the Patriot Act, more energy has been spent on preparation, including domestic surveillance.
The major impediments and roadblocks have to do with the lack of certainty as to what sort of terrorist activities might be taken and the need for multiple jurisdictions to be involved in the planning. Agencies essentially have to plan for everything, everywhere. They have to make these plans while consulting and coordinating with different states, towns, counties, and other governments. This is a serious problem which makes planning very difficult.
The beginning of a counter-terrorism plan is threat assessment. What is it you need to protect? Property, a facility, people, access to an area, etc. Then try to list the types of risks including specific groups targeting you or generic groups that might randomly pick you. Attempt to determine types of attack for each groups. Preventative measures consist of target hardening, providing boundaries (whether fences or security perimeter, firewalls for networks), increasing the type or quantity of surveillance. More active measures involve collecting information on the groups or individuals that are threats including how often and how much material should be collected and how it should be reviewed. The plan should include how to escalate notification of an active threat. The plan may include a list of technologies, items or personnel needed both to secure the facility or to collect information on threats. Finally the plan should anticipate how often it should need to be updated or reviewed.