Techniques. Is this in all areas of functioning, or just novel or specific situations? Kids know when something is hard for them by kg. "i can't" often avoids a task or direction. Break the task down into parts & say "i'll help you, but i won't do it for you, " if the task is age-appropriate. Ignore the protests; wait 2-3 minutes & calmly re-state "it's time for you to.."not yes, you can!" practice!
Yes. Confidence is not automatic. It stems from practice. So, the "behavior modification technique, " might be to reinforce practice behavior. If a child pursues something new and begins developing skills related to the pursuit he or she will develop confidence related to the pursuit and related tasks. If the child lacks confidence in interpersonal interactions, increased targeted practice helps.
Unconditional love . Praise love thanking god every day for the gift of this wonderful child will produce a confident child--my math prof dad would give students problems they could solve & build their confidence that they ere competent in math--many became math majors.