I'm launching on Product Hunt in June. What's one thing I should definitely do and one thing I should avoid?

One thing you should not do is pin all your hopes on a product hunt launch.

One thing you should do is launch your app before you launch on product hunt and build an audience so when you post it to product hunt you can tell all your customers to go upvote you like all the other top products do.

Here
It's all the list of tasks to be completed and tools needed for a successful launch on PH

https://docs.google.com/spreadsheets/d/1u3zpvZa3E5vKV1pAK3kA6CVh9DH_9Q_C0rCHwzOpgW0/edit?usp=drivesdk

You shouldn't focus all your energy on the product launch. It's a great one-time strategy to get some eyes on your product, but you should also focus on getting more consistent traffic and user sign-ups.
That being said, a great launch can do you a great favor but you shouldn't base your products future on the launch results.

Just my quick thoughts.

Make the most of the places where you can put the launch message on PH; without this you will struggle to gather the votes you need

My company also plans to launch in the summer. There was a dilemma - to do the launch in beta, or to finish the product?
decided to debug all the bugs and come out ready to make everything work! There is a professional audience on PH, for whom all the little things are important - in design, in usability, in creativity of presentation.

And it's important to gather an audience for your product after all. Otherwise how do you create a buzz around your launch)! We made social networks, we pour traffic there. Plus we actively write on Reddit, here, on Hacker News, on Twitter. Exchanging contacts and recommendations with PH members is just important!
I don't agree that it takes 10 minutes a day... as a rule, I have more - it's voting, commenting, creating my own discussions - yes, it takes time!

A few points -

Must Do: Start forming connections on PH community. Be active there.
Must Avoid: Don't ask people to register and vote for you when you're live. Let the votes be organic.

PS: Don't spend more than 15 minutes of your time on PH. PH launch may or may not be successful. Instead, focus on long-term growth strategy.

I made a list of how we prepared for our launch, hope it helps! https://blog.tally.so/product-hunt-launch/

I got this advice from friend of a friend that never launch it yourself. Instead connect with some Product hunter and ask him to launch it. share profit or something. But if you launch it for your self. It will be hard for you to keep up with hunters audience.

One guy that successfully launched on PH recently told me this:

Timing matters. If you appear first when ProductHunt's "new day" begin (around midnight Pacific Time), do everything that's within your power to get upvotes/etc. Get the snowball rolling. The reason: if you begin first, there's a decent chance you'll also stay first. As new people check out PH frontpage, they'll see you near the top, causing themto upvote you, etc.

I agree with what Lain says. I'd go a step further and say if this is your first venture forget about the idea of launching altogether. Ship anything. Then iterate and improve based on further research and feedback.

I've been developing my application for over 12 months and still have yet to list/launch it on PH. I found users through online forums and social media. I "launched" it as a post in a niche community on Reddit.

Have a look at the Upcoming section on PH. This allows others to follow before you launch. I imagine it's a decent way to gauge interest and refine the core product offering.

If I did use PH it would be once I have something more mature and polished to stand out from similar products. And I'd follow one of the few great guides going such as this:

https://www.notion.so/Checklist-for-Launching-on-Product-Hunt-c282188c52474d0d9d8c0ffb7e26de64