In Kim Klein's article on stopping a subtle decline in donations, she mentions the importance knowing your donor retention and attrition rates. You can figure donor retention (how many you have kept) and atttrition (how many you lost) easily using Fundraiser.
Both are calculated by taking all the donors you had in the previous calendar year and comparing which of those donors gave in the most current complete calendar year. In other words, which of the people who gave in 2014 also gave in 2015? Expressing that number as a percentage of loss gives you your attrition rate; expressed as a percentage of renewal gives you your retention rate.