I used a book called "The secrets of writing," which is a Bond book. It's for supporting children for 11+, Common Entrance etc.. It goes over different tasks ie, letters, diary entries, fictional narratives, playscripts etc.. It then gives you ideas as to improve your writing, ie, any memories you have, borrow and steal (use a story you know and make it your own), interrogate and interview one of the characters to generate ideas and details. It suggests key words that can prompt and structure your memores/ideas (my son does this) using who?, what?, where?, when?, why?,how? (using one hand - fingers and palm) and then, see?, smell?, hear?, taste?, touch? and feel? (using other hand - fingers and palm). I hope this makes sense - but this one really works well with my son.
We still use this book for homework sometimes, for him to remember how best to complete his work. For example, writing a letter as a character from a book and more recently, writing a diary entry.