I highly recommend Lauren Rivera's "Ivies, Extracurriculars, and Exclusion" (Research in Social Stratification and Mobility 2011).  Not only is the piece careful, edifying, and interesting; it's even emotionally affecting.  Learning more about elite hiring actually replaced my apathy toward elite firms with sour grapes: "I never wanted to work for you anyway!"  

Rivera's sample:
I examined hiring processes in three types of elite professional service firms: investment banks, law firms, and management consulting firms. These types of firms share important similarities, allowing for a robust comparison.

Her approach:
From 2006 to 2008, I conducted 120 interviews with professionals directly involved in undergraduate and graduate hiring decisions in top-tier firms in each of the three industries under study (i.e., 40 per industry).  Participants included hiring partners, managing directors, and mid-level employees who conduct interviews and screen resumes as well as human resource managers.

[...]

To supplement interviews with behavioral data, I conducted fieldwork within the recruiting department of one elite professional service firm over a period of nine months. My role was that of a participant observer.  Given my prior professional experience at a peer firm and in event planning, I was brought on as an unpaid "recruiting intern" to help plan and execute recruitment events...  I shadowed recruiters through the recruitment process for full-time and summer associate candidates from a single, elite professional school, debriefed interviewers on job candidates immediately following interviews, and sat in on group deliberations where candidates were discussed and ultimately selected.

Big results:

1. Most applications practically go straight in the trash.  
Because professionals balanced recruitment responsibilities with full-time client work, they often screened resumes while commuting to and from the office and client sites; in trains, planes, and taxis; frequently late at night and over take out... [E]valuators tended to do so very rapidly, typically bypassing cover letters (only about fifteen percent reported even looking...