When I think about building an athlete's self-confidence and coaches speaking about making athletes more confident, I remember something I was taught from a professor's dissertation on self-efficacy. What he stated, was that self-efficacy is derived from demonstrated ability. This does not come from a magical formula or a professional coaching book, but what this is screaming at coaches is clear and important: when an athlete succeeds, they will feel more confident and become more self-efficacious toward that given task. What coaches should take from this information is that we need to put athletes in positions to succeed, not fail.
I have often times seen a practice or drill that puts an athlete at an extreme disadvantage to complete a task successfully. Sometimes we play 1 v 1 and pit the biggest and strongest kid against one of the weaker ones(maybe not on purpose). What do you think will be the outcome? In addition, what are the feelings associated toward these atheltes following the outcome? I'm not advocating that coaches need to think about each individual's feelings when designing a practice plan because that would be quite overwhelming, but I think on a whole we need to be more conscious about the situations we are presenting atheltes. Try and become more creative in planning practices to include situations where success is likely. Athletes will respond by gaining self-efficacy for the given task and continue to develop that task until mastery.
Remember self-efficacy is also task-specific, meaning an athlete may express high self-efficacy making a breakout pass, but not necessarily a pass from the point etc.