How do you build great working relationship/s with your PM/s and other product stakeholders?

The answer here is investing in the relationship. A PM is a PMM's primary strategic partner in most cases. I advise my team to prioritize the weekly 1:1s or coffee chats. It's important connect at the human level, by being curious about personal and non-work parts of their life. A healthy working relationship is founded on mutual respect and empathy.

The start of building a great working relationship is clearly showing the expertise and value that you uniquely bring into the partnership with Product. As a PMM expert, you want to be seen as the central hub for a lot of critical information regarding not only your customers, but the products you sell, competitors, industry analysts, and many other constituencies. To be considered a subject matter expert, you need to continually gather and analyze data and business intelligence from all of the sources mentioned, Sales and CS, plus external sources — and use this data to inform the evolution of your recommendations around strategy and messaging. This is what separates great PMMs from good PMMs. Competitive intelligence and industry expertise tend to be weakness areas for Product that PMM can uniquely deliver, so that would be a good place to start to beef up your knowledge base.

Great question. Building relationships with Product is paramount to our success. I think this comes down to a few things...

1. Establish a personal relationship with your product stakeholders. Get to know them and what drives them (beyond just work). I had one PM I used to work with who is a musician and plays guitar when he's not working, and we always talked about music and connected much deeper than just the latest project. Beyond that, make sure you have regular 1:1s with your product stakeholders and connect on the projects they're working on, but also that you take an active part in the conversation to share what you're learning and hearing in the market. 
2. Be adaptable. There have been countless examples in my career when a release has shifted dates. Be adaptable with product, and ensure you are supporting them and planning on timelines that make sense for you both.
3. Provide strategic guidance and value. An important piece of building a productive relationship with your Product counterparts is proving the value you provide. This can mean collecting win/loss data and sharing insights on what product should take away, competitive intelligence, or virtually anything else. As a product marketer you should have unique insight into how your product works, customers, competitors, and the broader market and with that lens you have valuable perspective to provide to product (and virtually everyone else inside the organization).

I think I mentioned this in another answer, but I think a good framework is the Five Dysfunctions of a Team model. Build trust with your PM and other stakeholders--that's delivering results when called upon and being a key thought partner with your stakeholders. That will allow you to have productive conflict--which is about being comfortable enough to challenge an opinion because you already have mutual trust with your stakeholder. That will allow you to have buy-in on plans because youve both asked the hard questions. That will allow you to have accountability for what happened because you had previous alignment, which will drive results. 

Thats obviously a lot, but if you can really work on building trust with your PM--know your user, know your product, deliver results (make great decks, build awesome emails, produce better marketing results than the average) then your PM will seek out your opinion.

In some cases I've had recurring PM syncs, In some cases I havent. So its not so much about your working model or cadence, but more around how can you own your space and deliver results back to your stakeholders.