What is ONE piece of advice you have been told/read that has really improved your gameplay?
Or just a tip for ranked solo que.

Jungling. By far.

Have a thick skin. You're likely to get flamed a lot.

I'd also like to say "Don't listen to flamers", but only do that if you have an extremely thick skin, since you'll just get flamed more for ignoring them.

Wards win games. Even if you're the only one buying them, warding key areas will save your whole team. Conversely, if you're not dying, and map control is pretty even, pinks or Oracles will win you the game. Reliably.

In ranked games, do not set yourself up to DEPEND on a teammate for any situation. With lack of verbal communication, you might go to pursue and they might not follow or other such dilemas will arise leading to your death and causing heightened stress levels.

I also would say don't listen to flamers. If you have to respond to them, be OVERLY nice. Otherwise you feed the flame.

Blame yourself for everything.

It makes you a better player, and if you worry about becoming a better player instead of getting a better elo, your elo will go up

Patience.

Adapt to the players on what they are playing and how they react.

Use pings to alert teammates. As much as possible, good coordination is key to victory and pings are the easiest way to achieve that in solo games. People often don't read chat or watch the minimap, but its hard to ignore the pinging sound unless they have the sound turned off.

Use that ping to highlight the priority target in a fight.
Fall-back ping is great for calling off a gank when you can see that it will probably go badly.
I don't even type out MIAs, I quickly ping in a line leading to the lane that is probably getting ganked.

For teammates who completely ignore your warning pings, they are pretty much a lost cause. Focus on coordinating with the rest of your team.

Don't become stressed. I know if I am nervous or stressed I play worse.