At the end of the gangster era, the FBI was still in the midst of multiple training areas to better itself. By the latter part of the 1930s, FBI Headquarters had queried all SACs in the field for suggestions on the best way to conduct surveillances.  Needless to say, most were drawing their comments from their own experiences over the years, along with the men under them. In some cases, those with former positions in local law enforcement were able to supply additional experiences.   

While we don't have the ability to show all thirty something opinions here that eminated from the various SACs in the field, there was one common denominator that surfaced in all of them; common sense. 

Bureau supervisor, SA John Welles, composed a paper on surveillances in 1938 and it caught the eye of J. Edgar Hoover's Exec Committee of ranking Bureau officials.  In the end, it was set forth in the Bureau's manual of investigative operations along with being forwarded to the field as an "all SAC bulletin."

The paper of SA Welles and suggestions to the field is set forth here.  If readers spent any time as law enforcement officers, its content is well appreciated and understood.  You might even dub it "Surveillance 101." The comments near the end about "peeking around corners" and hiding behind telephone poles can only bring a smile...