Looking to upgrade your genealogy skills? There's a load of good advice in a series of articles online from the US Board for Certification of Genealogists. Some of the advice is US-specific: those of us in Canada the UK and elsewhere will have to adapt to our situation. Much of it is short and to the point.
Kathleen W Hinkley has an article on Analysing City Directories. She shows an example of different people with the same last name being found at an address in different years indicating family connections that would not have been apparent from a single directory. If you are have access to runs of city directories for a location of interest take the time to go systematically through the directories. Where did people live? Did they change the way the name was spelt, or the forename presented? How did the description of the occupation change?