Jade Craven who runs the blog, The Prolific Writer published an awesome list of tips the other day on how to get a guest blog post on an A-List blog. Her tips are bookmark worthy. I can verify that her tips on Waiting For An Opportunity as well as having prior contact with the A-List blogger work. I used most of the advice she provides in order to write guest blog posts for Problogger.
Definitely give this one a read and save it for future reference.