I can't find the best one I saw (which I believe said to set a default price), but I found these others:
http://www.freakonomics.com/2010/07/20/how-to-maximize-pay-w...
this one is long, but has a section on "pay what you wish" midway through.
http://conversionxl.com/pricing-experiments-you-might-not-kn...
(1) I mean something like saying: "you accomplished X goal this month !!!". Probably someone else has more specific advice :) , I've never tried pay-as-you wish myself, but seen lots of previous advice here!