One of my favorite things to do is tell a student that I really like how kind, helpful, thoughtful, funny, etc. they are, and watch them go "wow, I really am, aren't I?" and then look for more opportunities to be that, and think of themselves more kindly when they do. It doesn't have to be perfectly true. It can be like, you are so supportive of your friends, and that's why I know if I ask you to lead you'll do a good job. And then they do. And they feel awesome about it.
One of my other favorite things to do is this but with adults. I tell my parent I admire how emotionally vulnerable they are, and then they think about all the times they might have done that- times I maybe didn't actually see at all- and think yeah, I am, and that's a good thing! And then maybe that's a thing they actively try to be now, where before it may not have been something they cared at all to try to do. And maybe they love themselves a little more when they do, too, and that's a good thing.