Create a lyric dump - just a text file you use to collect phrases, lines, even single
words. Then when you start to write lyrics to a song go back through it and pull out all
the words/phrases/lines that have potential. Even if you don't use any of them it's a
great way to get the ball rolling, and you never know when that throwaway line you came up
with in the pub actually turns out to be the missing piece of the jigsaw. My lyric dump
has proved invaluable over the years, and it grows faster than I can use it all.
If you want to get really into it you can create a database, and search for
words/phrases/etc by rhyme or mood or whatever. The possibilities are endless...