I don't know if I believe in a Muse or if I believe in putting in the time and sometimes your practice pays off and what you're trying to say slips out ore easily. You have to put in the time and hope the Muse shows up.
Sometimes I don't believe in a Muse, but I rather believe in "Having Something to Say." I sometimes wish I had more to say, or that what I had to say wasn't so trite.
I can see how personifying my Muse could be useful in a magical, ritual way, because then I could use the Art of Changing Consciousness at Will and invoke the Muse consciously when I sit down to write. And it would be fun to build little pocket shrines to my Muse out of old pill tins.
I like the distinction Elizabeth Gilbert makes between "being a genius" and "having a genius."
I think it's dangerous to confine one's Muse to a single gender or age. If you had asked me in 1995, I would have said my he was a 40-something, French archaeologist/sociologist (or sometimes Marcus Cole). If you'd asked me in 1990, she was a two dimensional cartoon hippy woman. If you had asked me in 1985, I would have said my Muse was a speed-boat driving, femme fatale spy in a swimsuit (probably an extra from a Duran-Duran music video). These days, I think my Muse is a giant mug of tea. I guess as I get older, essentialist ways of thinking about sources of inspiration are less useful.
But sometimes, when the writing is numinous, I believe in a Muse.