I won't be specific as to techniques, partly because I don't really know. I can, however, offer some thoughts about possible terrorist strategy and our security responses to an organized terrorist threat and the likely freelance, loosely-affiliated cells that might spring up here and move to a drummer heard or imagined.
The first point to note is that we can withstand a lot. Witness 9-11 and before that the tumult of Vietnam, the sacrifices of WWII and the dislocations of the Great Depression. Nobody's going to take us down in a world as now configured. Thus, assuming a central intelligence ("al Qaeda Central," say, and bin Laden and Zawahiri) planning a true "war" on the US and the West, to the extent our defenses literally force them to take a "what opportunity offers" approach, we must assume that they would opt for a sustained campaign with a defined and achievable goal as their purpose. I will leave the details of that formulation to general discussion. I will note that one successful, spectacular, additional attack could send us reeling, but we'll get up and come back with renewed ferocity, and they know it. A kill-strike is beyond their capability, indeed anyone's as the world is now configured. One key is to line up in advance through diplomacy all "responsible parties," those with influence over terrorist policy and operations. As is now clear from events and hopefully is becoming clear from these thoughts, the emphasis going forward should be on US-smaller scale, pinpointed intelligence, military and law-enforcement strikes, and not on setting the fuse for a conflagration, if that can be avoided.
As to loosely-aligned cells, the danger they pose in a rational world (assuming they could not get true WMD materials without central, coordinated support) is similar to, but greater than judged by damage from an individual event, the curse of criminality we now deal with every day, and should be dealt with similarly: steeped up patrols, etc. in a security-conscious populace.