Like Dave, I get a hook and then try to surround it with a good story or song idea. If the hook and song idea aren't there, I don't take it any farther. I don't have enough time in a day to waste on bad song ideas. And since most of what I write is for me, the bar is pretty high - as like anyone else, I don't want to embarrass myself in public. My experience performing has made me a better writer as I now think as both the writer and performer even if the song is not for me to perform.