I can't storyboard. If I try, the page becomes already-done in my head, and then getting through it is like a death march. (Bear in mind I'm doing the art, too, so there's a good bit more freedom to skip the storyboarding than if I was working with an artist.)
Generally I write the dialog, and once in a blue moon, a stage direction, like "rolls eyes" or "charges" or "dangling from bridge." Then I pull up the page, read the dialog to see how it breaks down best in my head, and start dropping boxes into my photoshop template until I've got a layout that works for me. (It's been three years since I started doing this webcomic, so this goes pretty fast--I usually have a good idea what I can get away with.)
As an artist, the few times I've worked with other writers, I have never gotten a storyboard from a writer--I just get dialog and stage directions. Most of the time, they have a pretty good idea what they want per panel, and they describe it verbally, rather than trying to draw it.
They frequently also have no idea how much actually fits in a panel, so if you're working with an artist, let me say for the record that you can generally fit about two-thirds of the dialog and maybe half the detail you THINK you can fit. Err on the side of minimalism. Your artist will thank you.