Speak about something wonderful your child is good at or something great they did. But for an added boost, say it "behind their back" to someone else when you know the child will hear but does not think you are aware of their earshot.
My teenage daughter was in a surly mood one day and I spoke to my husband (right outside her bedroom door!) of how talented she is at art.
A few moments later she emerged happy and made me a cup of tea!