Christopher Adolph and Gary King. "A Consensus on Second-Stage Analyses in Ecological Inference Models," Political Analysis, 11, 1:86-94, copy at http://gking.harvard.edu/files/abs/akhs-abs.shtml. (Article: PDF)
Since Herron and Shotts (2003a; hereinafter HS), Adolph and King (2003;hereinafter AK), and Herron and Shotts (2003b; hereinafter HS2), the four of us have iterated many more times, learned a great deal, and arrived at a consensus on this issue. This paper describes our joint recommendations for how to run second-stage ecological regressions, and provides detailed analyses to back up our claims.
Also see related research.