They can never say that they don’t know what to write about anymore!
Encourage your students to keep a list of things they can write about in their writer’s notebooks (an idea I took from The No-Nonsense Guide to Teaching Writing by Judy Davis and Sharon E. Hill). Here’s a page out of a former student’s notebook. You can see the specificity grow as the list goes on!
I typically ask my students to add 2 – 3 new items/week. However, you can have kids do this at any pace you please.