How? Patience. I add things I plan to order to my cart, and once the collection goes above the "free ship" threshold (currently $25) only then do I place an order.
Of course, on the order page they always default to "paid shipping" and force one to explicitly check the "free shipping" radio button to actually get free shipping.
Some years back it felt like Amazon deliberately delayed for an extra week any "free shipping" packages -- they would sit, waiting, for about a week, then packed, shipped, and arrived in about 4 days. I always attributed it to Amazon punishing those who chose to gain free shipping without signing up for prime. But over the last few years that "delay" has shrunk such that it no longer seems like "free shipping" packages get intentionally delayed to "encourage" prime sign-up next time.