Writing - Our Fickle Mistress

Here's the thing we all know/love/hate about writing: it's a gift, but it's not really ours to wield. We have no control over this situation whatsoever. It comes and goes when it pleases. But... isn't that kind of the beauty of it?
No. No it is not.
Well. Okay, so maybe sometimes, when the bitterness has faded some and we're strung out on coffee/chocolate/other various sugars and a little hysterical, then it's kind of amusing in a giggly sort of way. But you know... other than that.
How do you cope with writing, that unstable mistress? What's it like to love something so fickle?

I'm referring of course, to the way you can set aside time to write - hours, days even - and plan it all out. Set up your writing station just so, stock up on caffeine and finger foods, turn off the phone... but it's like Julia Roberts in Pretty Woman. She's not at your beck and call. 
But then the second you're preoccupied with something ENTIRELY too important too ignore - that's when she demands your attention. 
Welcome to life, folks. 
It happens to me more often than not when I'm in my English classes. Or in any situation where math is involved. I'm sure I could analyze this further, but really the why isn't all that important. It's the "What now?" that really matters. 
I try to keep little notebooks on my person all day so I can just give in to the muse, let her have her way with me and cope with the outcome later. I also have little jotted notes ALL over my class notes. 
So tell me, how do things work for you? Do you ride the wave, or resist the flow and make it happen?