If it is at all possible that your subject may move, bolt, fly away, stop smiling, or just get tired of waiting for you to take the picture, shoot once right away.
Practice getting quicker and quicker to the draw.
Do not worry about taking too many pictures and do not wait until you're absolutely certain all the knobs and buttons are in their correct position.
Check out my Online Photography Courses to learn how to get quicker at capturing the best picture you can.