Don't give away any information

General tip for ranked games:

Don't give away any information to your opponents. You don't have to be a jerk, but you don't have to say anything.

If you used the wrong runes/masteries, don't announce that in all chat.

Don't give your laning opponent tips on how to play the game or itemize.

Don't tell the other team what skills they should max first.

Just don't tell them anything that could be used against you in the slightest.

Be nice in the postgame, after you win. You might not think that little things matter, but you really want to take every advantage you can. I've seen a n enemy jungle camp a lane after the player announced in /all he had the wrong runes. The player fed and we lost. I've had a top lane Rengar coach enemy Darius how to play against him after the Rengar drew first blood. Darius killed Rengar three times after that. We lost. I've had an ADC tell enemy Vayne that she should rush BotRK instead of IE after a pickaxe first buy. Vayne bought BotRK asap, won the lane, and we lost.

I still remember when teammates would announce dragons taken and give away timers. That's not relevant anymore but the point is the same. Just don't tell your opponents anything.