I have worked very hard to be a woman who is sincere in what she says and what she does. All talk and no action doesn't sit well with me. For me, being phony is not an option.
Still, we often forget that while phony may be the opposite of sincere, there are other places along the spectrum in between... and I think working up to being a fully sincere person is a great goal. It's not lofty, it will influence many other goals you attempt in your life, and it will aid you in being an authentic person through and through.
So often, people talk about how they wish they were more this or that... but you don't ever hear someone say they wish they were more sincere. Sincerity isn't even on the radar much of the time. It should be.
Mark Twain once said, "Any emotion, if it is sincere, is involuntary." Think about that. Let it stay with you.
Being sincere with others in your life is important, but being sincere with yourself is utterly vital.
Don't work hard at being sincere. Just be. And the sincerity will be there too.