Probably the most important ...

Probably the most important aspect of individual defensive play is the 'close-out.' This approach to the ball should be made in a 'step-drag action' with advanced foot moving forward first and then the back foot in a boxer's type shuffling of the feet.
 Tex Winter

Quotes from the same author

The main thing is, too many players feel like they're complete players when they're not, so rather than acknowledge their limitations and play within their ability, they overreach.
 Tex Winter