A note book.
If you hear or think of an interesting phrase write it down. Any odd bits of lyrics that come to you , write them down. Sometimes more structured ideas will come to you, write them down. When you come to actually finalise your song's lyrics those notes can be a useful resource for filling in gaps when you just need a line to finish a verse etc.
I've kept many such note books over the years and it's not uncommon for me to use a phrase I jotted down ten years ago in a song I'm writing now .
I Sometimes make notes just brainstorming ideas for months on a theme before moulding them into lyrics.
The current A4 book showing the notes, rough work and out takes from a recently finished song. I had the original idea about ten years ago and finally got it done a few months back after a few false starts. It's a silly country and western song.
The book is filling up with pages of similar gibberish that may or may not find it's way into a song.
A thesaurus and rhyming dictionary are useful too.