I'm in the middle (taking a query letter re-tooling break) of querying my manuscript with agents.  I'm curious about any published or represented authors have any tales about how many query letters sent.  Did you change your letter or book after the first rejection?  The first 10 rejections?  Only 4 agents in myself (3 rejections, one twice - they asked to see query letter again - 1 not yet replied).  Any advice aside from the traditional "be persistent" and "write a firecracker query letter"?

Thanks in advance, really interested in hearing some anecdotes.