I know I haven't been a staff member of anything here yet, but what I think happens is that the higher-ups (aka smods and admins) discuss candidates for certain areas based on their activity and contribution to the section, and then take a vote and if successful the user in question gets promoted and they usually tend to do this in pretty fun ways, ranging from invitations to Skype convos or even spamming emoticons via VMs, and stuff. I know they tend to do weird things when they are promoting a new mod.
But yeah, you have to be active in a board you feel comfortable in and if said board is in need of a mod there's a chance you may be up for consideration. I know join date and postcount doesn't matter, since I've seen a few cases of those with a fairly recent join date and not a huge postcount.