I would sometimes fill in for finance on a rainy day, and even though I was top 0.3% in the country when it came to car sales, I Couldn't sell that garbage even with a gun pointed at me. I just can't lie like that.
As a basic set of rules:
a) say no to everything.
b) look at the service intervals in the book (or what the maintenance minder in the car says).
c) ask them if the factory service plan would cover it
d) ask them if it's critical to safety, and if so, explain what would happen if you didn't do it, how you would notice it was an issue, and how you should respond while it's happening on the road.
e) as soon as you run out of factory service, and maybe factory warranty, use an independent mechanic
Now, sometimes the factory service intervals are wrong, and sometimes there's stuff you probably should do that the factory doesn't say to do ever; but for the most part, the factory schedule is a good baseline, and dealer service wants to do extra frivolous stuff. Some independents do too, I have never seen a next oil change sticker printed with factory intervals.
For example the green energy clipboard person trying to get you to switch power companies, you can tell them that you rent and there is no comeback.