A small notebook - one you like and don’t mind carrying around much of the time - is a useful companion. Salvage those scraps of imagery. Dreams, daydreams, musing thoughts, reactions to things around you - a billboard you saw with a picture that reminded you of a long-lapsed friendship, a song on the radio that back schooldays - these can often be mined for a sentence or two.
When writing stalls, this notebook is handy for browsing. You’ll find you can decant from it the stimulus you need to go forward with idea.