I have Imposter Syndrome. I've been in the business for 20+ years, command a large salary, write this blog, teach scrum and process management and I still feel like someday, someone's going to notice I have no idea what I'm doing and that I'm just making it up as I go along.
Turns out, knowing what your doing is a fluid thing; making it up as you go along is perfectly acceptable and has gotten many people to great heights of accomplishment. Albert Einstein rarely remembered to tie his shoes--he went to work in slippers when his wife was out of town--but we're still quoting him today.
I am, by no means, Albert Einstein or anyone like him. But I am an expert at what I do. So are you. Think about it a minute. Strip away the "wait until they find out I don't know what I'm doing" and think about your job and the accomplishments in your life that led you to where you are. Got it? See it?
Yeah, you're a freaking expert.
You would be paid to do what you do if you weren't; from flipping burgers to writing code (and everything in between) employers are not just hiring people who can get by in this economy, they're hiring good, resourceful people. Experts, or soon to be experts. You're one of them.
When you look at the folks you manage, put the expert glasses on and take a look. I bet that each member of the teams you work with or the team you manage has something you'd consider them the lead on. Something that is in their wheelhouse of expertise. People often forget that they were hired because they rocked before they got this gig. They often get bogged down in the minutia of what they are doing and how well (or how poorly) that particular minutia is doing. Even if you know they could do more than they are doing now, you know it because you hired them to be competent. You hired them for their expertise.
People you work with need to hear that now and again. Not just compliments and praise and what have you, but that they are experts. That they were hired to do the job they are doing and they are doing it well. You need to hear that, too, incidentally. The entire department may have left for another company (which actually happened to me one time), leaving you the senior one in charge, but you're the one with the l33t skillz. You deserve to be there, both because of what you know and maybe because no one knows anymore than you do.
In any case, feel self-doubt: you will. Listen to your employees kabitz about their problems and their work. Then remember, you're a freaking expert. A professional. You're getting paid to do this. And so are they. Sometimes this means they need to straighten up and fly right to earn that concept of expertise, but typically, they just need to be reminded of it in their darker moments: that you rely on them because they are experts.