The phrasing of the question -- "what are the most effective investigative techniques in conducting an intelligence-driven investigation..." -- is somewhat tautological. The key to bringing down any large criminal organization is the collection of intelligence. Without it, there is no investigation. Intelligence derived from informants, wiretaps, forensic evidence, interrogations of suspects, etc., is the key to any criminal investigation, especially one involving drug trafficking organizations or cartels. All of these components are required for an effective prosecution to take place.
In the case of domestic drug trafficking organizations, a great deal of manpower, time and financial resources are required for an effective investigation. The most crucial information comes from wiretaps, when suspects are unaware that their conversations are being recorded by law enforcement agencies, and from well-placed informants. The latter, informants, are generally comprised of two categories: criminals or associates of criminal organizations who decided to help law enforcement by secretly conveying information to police, and undercover law enforcement agents, in this case, primarily the Drug Enforcement Administration (DEA).
Of those two categories, undercover law enforcement agents are more prized because their credibility is considered far greater to attorneys and juries alike. Criminals who agree to cooperate with police by betraying their associates are considered less credible because in most instances their decision to cooperate with law enforcement is part of an agreement that their own crimes won't be prosecuted, or that they will be given shorter prison sentences in exchange for their cooperation.
Wiretaps, however, trump all other sources of information. They provide first-hand evidence of a criminal conspiracy. The criminals are prosecuted on the basis of their own words, and not those of an informant. Wiretaps, however, require the consent of a judge, who requires "probable cause" that a crime is involved.
In the case of foreign-based drug trafficking organizations, all of these sources of intelligence are just as important. Because the investigation is occurring outside of the United States, and because it involves foreign nationals, additional U.S. Government resources can be brought into the investigation. In the case of electronic eavesdropping into the conversations of drug traffickers, especially the leaders of the organizations, the U.S. National Security Agency and other agencies may be utilized. Such was the case in the investigation into the late Pablo Escobar, who was the most important Colombian drug trafficker operating out of the city of Medellin (hence, the Medellin Cartel).
Another very important component of an intelligence-driven investigation involves the money trail. Drug trafficking involves huge sums of cash that is difficult for the traffickers to hide within the legitimate banking system. Tracking the flow of the money, therefore, provides useful information on the structure and practices of the traffickers. Money derived from criminal enterprises has to be "laundered" so that it appears "clean." That requires the cooperation of corrupt banking officials and the transfer of funds through electronic means or simply by what is known as "bulk cash" shipments -- in effect, suitcases full of cash. The more police know about the movement of the money, the better the investigation.