Can I recommend the neglected art of letter writing? (OK, it doesn't have to be snail mail - by 'letter' I mean anything written letter-style.)
Even if you assume nobody's interested in your startup (and why should anyone care that you or I have started a business?), presumably the problem you're solving for people is interesting. And presumably you have and are adding to your knowledge about the domain of that problem.
So read publications that take letters and bloggers who quote correspondence. Look out for places where your expertise gives you a fresh angle on a story which is somehow incomplete - maybe it's controversial, maybe the writer is quoting a variety of opinions but leaving the topic open, maybe there are specific questions posed.
So now you have the raw material for a letter. Pare back your opinion to the essentials - and work out how the person who reviews it will be able easily to check that you have credentials to back what you say. Provide simple information. And don't plug your startup or pitch your proposition.
Like press releases, letters are frustrating. You can write a dozen fantastic letters and see nothing published. But if you persist, letters are a way of getting onto a writer's map, each one can be fresh and topical, and you're showing how you value the writer because you've read what they've written and thought about it.
This isn't an alternative to persisting and refining releases of whatever form. But it is something that works well side-by-side, and especially in the periods where, if you're honest, you have nothing new to say about your startup.