One “this is good” scribbled in the margin has kept more kids going than any poster in the hallway.

Specific praise from one person beats a general message to everybody. A poster is aimed at no one and reads as decoration. Three words in the margin of your own work are aimed at you, about something you actually did, from somebody who had no obligation to write anything. That is why people remember the teacher who did it and not a single slogan on a wall.

You can do this for other people long before you are anybody official. Tell a friend which part of their essay or their game was good, and be exact about it. General encouragement slides off, and one specific sentence tends to stay.