It seems like people in product marketing come from all different departments, yet it can be hard to interview & make that functional transfer breaking into product marketing. What tips would you share with someone who is looking to breaking into product marketing?

You are already one step ahead about narrowing down your next move to product marketing - congratulations! 

1. Product marketing roles and responsibilities are different with every company. I'd encourage you to start talking to your target company PMMs and your current company PMMs to get a better sense of their day-to-day. 
2. Start aligning your current projects to map to the PMM responsibilities. For example: see if there is an opportunity to partner with your organization's PMM team on a common project. If you are from the Sales organization, partner with PMM to put together a customer-facing sales asset. That is resume-worthy and could be a good test to see if you like the PMM function. 
3. Be transparent with your leadership about your career goals, so they can also help align you with the right teams when the timing is right. 

Good luck with your move!

I wrote a post a while back about getting a job in Product Marketing that is still relevant and helpful. Overall though, think about your transferrable skills. Whether that's project leadership, storytelling, market intelligence and how it can apply to the job description of the PMM role you're looking at. 

Everyone wants to see experience, but if you don't yet have formal product marketing experience think about volunteering for a project to build that muscle (and ensure you like doing the work).