My mentor actually suggesting thinking of it as a funnel cone or kitchen sieve. Start with the broadest topic and continually narrow in until you reach your finite point. You could also think of it as a sort of taxonomy, starting out by identifying the "big idea" and then working your way to a specifically named topic. My peer review writing group actually drew cones on paper and filled them in with the logical steps until our arguments narrowed. Following those steps really helped in writing the lit review and kept me on a logical track that focused to my main point.

Hope that helps,
Nic