I think the proverbial "be yourself" is intimately connected to the proverbial "know thyself".
So it's not even about the image one has of oneself. Definitely not about the clothes your wear or your interests....but about being able to throw aways stupid labels like "I'm sexy, I'm cool, I"m a leader,I'm a failure,I'm geek,I'm a jock,I'm a self-made man, I'm a bum etc" and just BE beyond labels and boxes and other people's interpretation and our own interpretations of who we are.
It's also about being a dick, if that's how you really feel inside, or being a loving person if that's how you feel inside. It's about non-judging and not taking façades too seriously and also about facing your inner demons. Being able to admit your afraid when you are, being able to admit your biggest faults.
It's really all about transparency without the heavy layer of dramatic "i'm gonna show the world who I really am now!!!". It's not about being special. It's about being who you are without labeling everything as either good or bad.
At least that's my take on it. It's not about personal pride or about feeling ashamed. It's about being who you are rather than interpreting who you are.
For me a good example of a situation that can clarify this issue is about people coming out of the closet and admitting they are gay. For some people this means a period of gay pride, and going to gay things, and identifying with gay causes and dressing differently etc etc. But at the end of the day gay is nothing to be proud of, or ashamed. Being gay is nothing something you need to DO. You either are or aren't. The same can be said for anything else. Being gay is not wearing this or that or being like this or like that. It's what's beyond the social labels, pro or against.
Our society is SO obsessed with image and style over substance it's fucking ridiculous. Who you are doesn't need to be communicated all the time. Actions speak louder than words, so just be. Obviously that takes guts, but that's what the expression "be yourself" is calling us to do, imo.