I was lucky at the beginning of my PMM career, because I got into a pretty solid group that understood exactly what PMM did and didn't do. But I think my advice to people would be to really understand and grasp the craft of product marketing. We're not just writers - if we wanted to do that, we would have been writiers. Be a product marketer! This is someone who understands the target market (segments, buyers, changes, competitors), understands how people purchase your product in that market (all of the departments involved, people, time frames, purchase processes), understands the product technically (what does it do and what does it not do, be able to actually demo the product), have a solid go to market strategy, know how to launch different product types, and then be able to wrap all of that into cohesive messaging that positions that product, to the right buyer, in the right market, at the right time. Too often PMMs tend to focus in on just ONE of those areas, like writing messaging that they "think" sounds right, without having done any work to back it up.