When I am empty of ideas (and everything else) I turn to God's Word. By that I don't mean that I read the Bible for song ideas. But they always show up when I am reading. I start in Genesis in January, and make my way through the Bible, doing my best to finish by the year's end. I find help, strength, joy, peace, understanding... and song ideas. I'm not consciously "trolling" for them the way I sometimes do on Twitter or Facebook, but they begin to reveal themselves as I read consistently. When an idea shows up, I write it in the margins, sometimes even beginning the lyric there, and sometimes it's fun to leaf back through the pages and see how something I jotted down later became a song.
Here's picture of a page from Mark 14 from last year. Jesus was on trial, and the high priest asked if He was the Christ. Jesus answered, "I am; and you shall see the Son of Man sitting at the right hand of power, and coming with the clouds of Heaven." (Mark 14:62)
I don't fully appreciate what "Son of Man" means every time Jesus used it, but I know it was His favorite description of Himself. So I began writing in the margins about how amazing it is to have the Son of Man seated at the Father's right hand interceding for us. Later I finished the song with David Moffitt and Travis Cottrell.
My point is:
Find what inspires you and return there often.
Find a way to keep your ideas when they show up.
Make enough of a start on the writing to capture the inspiration. If you only write down the title, thinking you'll remember how and why those words moved you, it might be hard to get back to that place in your mind and heart.
Live like a songwriter, looking for your next idea to show up at any moment.
Here's the finished song: