I am working on a topical sermon for this weekend and at the moment my basic structure is a longish introduction with the rest of the material clustered around four questions: what? when? why? and who?
Maybe you are aware of Rudyard Kipling’s poem:
I have six honest serving men
They taught me all I knew
I call them What and Where and When
And How and Why and Who
Obviously his application is much wider than preaching, but if you are preaching a topical sermon, you may find Kipling’s “honest serving men” quite helpful.