If, while working on one article, I come up with an interesting idea for a future article, I write it down immediately and then return to my current project. That way I always have a stash of ideas in various stages of completion to work on.
This is key! I come up with so many ideas while writing a draft. I’ve also found it’s really helpful to go back over recently published work after a few days or a week. I almost always have ideas for a follow-up piece, an expanded look at some part of it, or some other related post.