Ideas use to come spontaneously. When I have the impression that a particular idea might make a new song or song text I write it down. I don't use a scrap book any more but a scrap file on the computer. When I'm at work I just send an email home so the idea is not lost. When there is no computer handy I write it down and key it into my file later. Some text fragments have been in this file since ages.
My file even contained a few completed lyrics, which I simply didn't dare to show to other people. Naughty little lyrics I mean which I had fun writing when I felt like being naughty. Meanwhile I've published them on an inconspicious page on my lyrics server. Maybe one day a young guy will find it cool singing stuff like "shut up" (do not look it up, you probably won't like it). If I don't sing it myself I need not feel inhibited. Rappers sing worse stuff all the time.
Most ideas just consist of a single line I like, e.g. "there's more to her than meets the eye". I would like to make a complete text around this one someday. Other ideas could make a part of a song, maybe even a chorus:
the door is ajar
why don't you come in
to enter my life
and keep staying within
The following text has rested in my scrap file for about three our four years:
don't save your life for later
'cause later may not come
don't cancel your own story
before it has begun
I won't pursue it any further because my latest text "terminal disease" covers exactly the same subject. And the following line actually has come out quite similar without consulting the scrap file:
...
never quit before you have begun
Some ideas simply stick until they sort of burst out.