I have found the best thing that you can do to a child is help them find a win, and then make a big deal about it. I cheer them on like the loudest, wackiest cheerleader that I can be and then hope that my excitement is infectious. Today a student was called on unexpectedly by the general education teacher and after he shared is answer he followed with, "but I think I'm wrong"... but he wasn't! It was one of the harder concepts that we cover this year and he had the right answer! I was so excited for him and I made sure he knew it, and you know what... it made him happy too. And after that he raised his hand to voluntarily answer a few problems which is something he hasn't done all year.
Now not every situation is as easy as that. Also that one moment of triumph is not going to take a student who is constantly disappointed in themselves and rebuild their confidence, but I've found if you can string together some wins fairly close together and let the student know how awesome that they're doing; that it can lead to a boost in confidence, even if it only lasts a few weeks.
I wanted to share with you a file that I use to give my student notes of praise.