Since I published Red Mercury nearly two years ago, I've had a number of people ask me "Where do you find the time to write?" or "I wouldn't even know how to start..." My answer to this question is maybe a bit smart-arsed, but it's true.
Get a pencil and a notebook. Write stuff down.
The more I've kept at it, through the writing of Neptune Rising and the as-yet-unnamed "difficult third novel", it's become clear to me the single most important thing you can do is to decide you're really going to write and then just do it. Whenever you can.
Personally, I've stopped beating myself up if I don't write something every day. I try to, and I get grumpy if I'm falling behind in my self-set timeline for getting a book done, but I found forcing myself to do it when I was really not in the mood didn't work out well. I invariably had to rewrite most of the stuff I'd done when I wasn't up for it.
So how do I write?
I write long-hand in pencil in a hardback notebook. I only use the right-hand pages, scribbling little additions on the left-hand pages as the evolving story might require. I try not to worry too much about finding the perfect words at this stage - it's better just to get it out.
Writing long-hand in notebooks means you can do it anywhere. Even in a spare ten minutes when it wouldn't be practical to fire up the laptop.
Once I have about 5000 words or so written long-hand, I'll type it up. The great thing about doing it this way is you enter into a kind of semi-editing state, where you're tightening things up and giving it some polish as you type. You end up with a first draft that's really kind of a "Draft 1.5"
The important thing is not get sucked into proper editing at this stage. I did a load of this with Red Mercury and it really held me back - jumping back and forth between writing and editing. Not productive. Now, I've got much better at concentating on just getting the stuff out of my head and onto the page - I figure I can fix it later.
When the first draft is done, I'll leave it alone for as long as I can resist, usually three or four weeks. Then the real work starts. As somebody said - "there's no such thing as good writing, only good rewriting".
But rewriting and editing is a post for another time...