I am tall enough that shrinking t-shirts is a constant annoyance! (though I have to admit I haven't ever tried the 'conditioner and water' trick, even though I've heard of it before).

Low temperature washes and avoiding tumble dryers works. I've also noticed thicker material t-shirts seem to definitely shrink a lot less! Much thinner cottton t-shirts seem to shrink a lot more, my mental model is that there's less material so when it bunches together to it's "happy place", it ends up a lot smaller. I have no evidence for this though.

Any other tips from people here? Also, has anyone actually tried stretching with hair conditioner?

I'm with you 100% -- shrinking tees and shrinking sleeves were the bane of my existence, I'd buy my proper size but they would only be wearable like 5 times until they got too short. But if I bought a size up, I'd be swimming in them horizontally even after they shrank. (The people here saying that shrinkage isn't a problem anymore, I have no idea what they're talking about. Maybe they wear a baggy/long style of clothing so it doesn't matter? And I don't care how supposedly "preshrunk" cotton is, it's not preshrunk enough.)

Now I just wash on cold and hang dry all my cotton shirts, tees and button-ups. Just use a folding drying rack as simple as this:

https://www.ikea.com/us/en/p/mulig-drying-rack-indoor-outdoo...

It's a little annoying to have to leave the rack out in the middle of some room to dry overnight, but zero shrinkage ever. The way it fit in the store is the way it still fits three years later.

And no, stretching with conditioner/shampoo doesn't work, because there's no easy way to stretch it the "right" way -- as you tug on spots at the neck and the waist to pull them apart, they stretch but in weird, inconsistent, lumpy ways. The final result just looks like you've had small kids trying to hang from different spots on your shirt and it's all out of shape. Maybe in theory if you had some kind of stretching system with long clamps or something it could work, but who has that? Doing it by hand, it's definitely not a solution.

Higher rise pants help. I have a long torso and started buying higher rise pants for the aesthetic difference, but they also make me less concerned about a t shirt becoming a belly shirt

You can use a dryer, just don't get all the way dry. Low heat until the shirt is 'damp', then hang to finish drying is what I used to do.

Hang-dry your tees. It's a slight annoyance vs just bombing everything into the dryer, but it's very worth it to not have sleeves that are too short. I usually hang mine on the shower's curtain rod to dry.

And frankly, this seems like less effort than trying to apply some hack to unshrink them after the damage is done.

Yes. It works. I bought my wife’s cashmere jumper back from child-sized. Pins, a sheet of ply, and a bunch of time.