Good sources:

• http://venturebeat.com/2010/08/16/beware-the-trappings-of-liquidation-preference/ (good source)
• http://www.theventurealley.com/understanding-vc-financings-liqudation-preferences/ (by DLA, has charts)
• http://fullratchet.net/episode-10-the-term-sheet-feat-brad-feld/
• http://www.feld.com/archives/2005/01/term-sheet-liquidation-preference.html
• http://billpayne.com/2011/03/16/the-liquidation-preferences-debate.html