One of the most understated but true advice is "Be yourself." While all (wo)men are created equal, the fact is, we're all the way we are, and there's no need to change ourselves and pretend just to "fit in."
As a writer, actor and artist, however, I find the advice a contradiction to my work. As an actor, I play roles. I pretend to be someone else. I put myself into some character's mind and pretend I have a whole different life and experiences and even personalities. How can I "be myself"?
As a writer, I create characters and stories so unlike myself, and I often have to put myself in their shoes, to understand what they're like, to feel and think how they feel and think. My stories also take me and my characters to worlds far away or created as a figment of my imagination. How can I "be myself"?
It turns out you CAN be yourself as an actor or writer or musician or artist without sacrificing your marketability. Often, the best actors don't just disappear into their characters; they also put themselves into these characters, and their uniqueness comes through whether they're playing a crook or a god. Take Justin Hoffman for example, arguably one of the best actors of our time. Whether he was playing Benjamin in Mrs. Robinson or Ratso in Midnight Cowboy or Tootsie, he made us believe in the characters and the lives they lead, and yet, he was also always so Hoffman-like, and you can't imagine anyone else playing those roles because "that's Dustin Hoffman!"
As writers, we also are able to put our author's touches and personalities and voices in there, especially when we write in 3rd person. In first person or 3rd limited, however, it can be tricky. How much do your characters sound like you?! And do your characters have the same values, views and knowledge as you, the author, do? It is indeed tricky, and can only improve with time and practice.
Still, I think it's important we need to be ourselves and understand our own voices and how we do things, whether it's acting, writing, singing, or whatever. Not everyone sounds like Josh Groban, and they shouldn't try to imitate. I enjoy John Mayer just as much as I enjoy Groban, because both of them have found their own voices (and what vastly different ones, too) and are able to be themselves. When an artist comes across as genuine instead of trying to be something he or she isn't, it becomes highly enjoyable and you appreciate them more.