From your experience, what are some best practices for product managers and product marketing managers to work together effectively?

Some best practices that helped my teams create a collaborate cross pollinating environment are:

• Standing 1:1 with PM and PMM leads to discuss major open actions with full disclosure
• PM contributes or leads customer feedback in regular product backlog grooming sessions
• PMM develops self service customer intelligence that PM can use to steer product strategy
• PMM leads customer advocacy efforts and invites PM and Eng leads to participate "hear from the horses mouth" 
• PM is included in Product PI planning process with PM and Eng

Trust is the most important factor in a PM / PMM relationship.

When I was a PMM, I worked with many Product Managers who did not understand the role of a PMM. They believed that the PMMs only role was to communicate the launch of a feature. This misconception often stems from the fact that in some teams PMs doesn't see value beyond the feature launch and PMMs do little to dispel the notion. 

So, how do PMMs and PMs build trust and work more effectively?

1. PMMs need to clarify their role. This sometimes depends on the leader's ability to communicate their team's functions, however individual PMMs can do a lot to articulate their role in the partnership. For example, PMMs can ask to be included in the discovery phase of feature development and bring relevant data and insights to the conversation.
2. PMs must communicate their roadmap early and often. On the flip side a PM needs to ensure they are communicating their roadmap, timelines, and feature delivery. If there are delays or changes, they need to ensure that the PMM is kept aware. Not only does this ensure there aren't miscommunications, it also builds long-term trust
3. PMMs need to build credibility. The best way for a junior or new PMM to build trust is to show their PM partner that they are a reliable. What does that mean? If you commit to a deadline, make sure you deliver. 
4. PMs should treat PMMs as equal partners. Some PMs see their PMM partners as "service providers" or specifically the feature communication team. This minimizes the role of the PMM and it also breeds resentment. PMMs need to show their value , but PMs also need to be respectful of the role a PMM has to play in the team.
5. Open and transparent communication. As a PMM you should be comfortable talking to your PM about anything (positive and negative). If you don't have that kind of relationship, you won't be able to work together effectively.