My co-founder Leo wrote a great guest post (could you have guessed?) on how to get started with guest blogging :-) Here it is - http://askaaronlee.com/bufferapp/

Also he wrote an article for OnStartups about getting press, which might also be useful - http://onstartups.com/tabid/3339/bid/80121/How-To-Get-Media-...

Hope that helps :-) Good luck.