I've been ped over for a promotion -- again.?

I would forget about the promotion in the short term and simply ask your boss what you need to work on to be a better employee. I manage people and while I understand your position, people who keep asking for promotions or more responsibilities have framed their goal in a selfish way - what more can they get out of the company. If you instead change your focus on what you can improve on in your current role then your focus seems a lot more selfless - even if they have the same end result down the road. Asking what you can do better is a lot less accusatory and is more likely to start an open dialog than "why didn't I get the promotion?"