I'm drafting business emails and constantly second-guess myself on when to use 'who' versus 'whom' in sentences like 'Who/Whom should I contact for approval?' What's the correct rule, and are there any practical tips to avoid mistakes in fast-paced writing?

I've reviewed some online grammar resources, but the explanations often feel too abstract. In my daily work, I write emails quickly and tend to default to 'who' even when unsure. For example, in phrases like 'The person who/whom I spoke to yesterday,' I struggle to apply the rule effectively. I'm looking for a straightforward method to check usage without slowing down my workflow.