Thank you to Scicurious for hosting a carnival on this subject.
As a woman in engineering, I expected to feel impostor syndrome quite strongly, but so far, I just haven't. I can find myself in a room as the only woman, and I barely notice. Am I some magical super-confident oddity? Not really. But even if I don't think I'm the best at ___, I still feel like I deserve to be where I'm at most days.
As I've been reading some of the brilliant posts around the internet, it seems that authors largely find in one of two categories: "best students" who no longer are, or people making a large leap from their background.
I've talked before about my SciGrandmother, and my other family members in STEM fields both in and out of academia. Clearly, I'm not making a large leap away from my background. Even at GiantU, I have a family support network, in the form of an aunt who is a professor in a biological field. She helped me find a mentor in my own department to help me deal with some of the nonsense that is my advisor. Having someone who is not a formal chain-of-command mentor has been incredibly helpful. When my own advisor was editing at the rate of molasses, he was willing to read my manuscript and give me feedback about organization and writing style, even if he didn't know all of my science.
As far as the "best student" thing goes, I've never been the best student at everything, thanks to my sister. It's been a life-long competition, with me winning some subjects and her winning others. We're fairly close in age, and very even in ability. I've had years to get used to having someone who was better than me. I'm better at math, she's better at writing. I'm better at music, she's better at drawing. And eventually, I've gotten to the point where that's ok. The best days are when one of us can say to the other "It's so helpful having a sister who's better at ___ than I am".
Lastly, I'm often just too damn busy think "I need to do x, y, z, alpha..." to think I'm not good enough. If I can't do everything, it's probably because there's so much of it. Every so often, I lapse into thinking I'm incompetent because I haven't finished everything, and then I vent to a lab mate, and they remind me I'm working on 3+ projects.
So what advice can I offer others? Find and cultivate a support network of various stages and fields. Find a peer you can vent with, find a mentor who's not your advisor (admittedly, this can be very tricky), make friends in other departments and other groups. Find someone who will tell you that what you do is awesome (that isn't a family member). Have trouble reaching out to people? Join Twitter, read blogs, form a virtual support network. Seriously, the blogosphere is full of supportive people from all sorts of career stages, even if it's dominated by biologists.