I keep a list of ideas for articles in a pocket notebook (and a bigger list online). Most of the ideas go nowhere, but I've found that scanning the list when I'm stuck on a piece is usually enough to get me writing on something.

Edited to say that my ideas list was inspired by Steven Johnson's spark file (https://medium.com/the-writers-room/the-spark-file-8d6e7df7ae58). Thanks to Kawandeep Virdee for reminding me in their response!