MODULATING FROM ONE KEY TO ANOTHER

RULE NUMBER ONE--

establish hook as soon as possible in new key, or in transition to new key. ideally, some repetition in short term, which is arguably the quickest way to establish a hook for listener.

(It may be true that there are other ways to establish a hook, but that those ways are harder to quantify or link to a formula. Thus, repetition is not necessary, but it's as close to a fail-safe plan as anything I can think of.)