writer's platform building campaign started this past Monday and runs until October 31. (Formerly known as a "crusade" Harrie has thankfully dropped that word and all its religious connotations which she never meant to invoke.)
The instructions are somewhat long and tangled, but once you've signed up, you take the time to follow all the other campaigners and spend time every week hopping around to the blogs of those in your "group" helping to increase their traffic but more importantly giving them feedback on their posts and ideally finding several writers who you'd like to follow after the campaign is over.
This spring's campaign, which I participated in, was insanely popular. Sign up for this round lasts through the end of August, so better hurry over if you're interested!