What are the most useful skills to have at the entry level? What job should you have right before you transition to a product marketing role?

Coaching/mentoring is my favorite part of the job. But there are certain pre-requisite skills I don't want to spend time teaching you. We could probably come up with a really long list of those, but in the interest of keeping this list useful, I will stick to three. 

• Be an excellent writer. For your first year or two as a PMM, your life is going to be really tactical. I typically task entry level PMMs with a lot of ghostwriting. Whitepapers, blog posts, sales blasts, campaign copy, etc. Great writing takes a lot of practice. You can learn it by doing, or let it atrophy by not doing. Write for fun and you will be far ahead of most junior PMMs. 
• Be at least conversant in your market. Find a good news aggregator for your industry (in cybersecurity, I love the cyberwire) and scan the summary every day and try to read at least four or five articles a day. If you're in B2B and have access to a Gartner or Forrester subscription, read as much content as you can get your hands on. Try to find communities of your buyers on reddit, spiceworks, etc and soak up what they are saying. 
• Get good at talking to people. If you're shy and/or introverted, this might not be the right job for you. Whether you're a PM or a PMM, you need to spend a lot of time talking to and understanding buyers and users. So if you're not really good at probing, discovering, asking second and third level questions, you need to get good at it quickly.