I've always found the best inspiration to be a deadline. "Oh, the show starts rehearsals in a week; better finish that song!"
Seriously, sometimes it does help to get singers/actors involved. Set a date to have someone learn a new piece and you will finish it in time. Writing is all about making choices. When you have less time to dwell on your choices, your first instinct is often the right one.
Also, Doug has a good point. Take a breath, clear your head, meditate. Go for a walk. I often ride the subway. The loud noise drowns out the noise in my head and often the work I'm doing resolves itself. Just my opinion!
"It's not so much do what you like, as it is that you like what you do." SS
"Time flies like an arrow, fruit flies like a banana." GMarx