Simplify a part of defensive thinking as maintaining and enhancing BALL PRESSURE.
With individual assignment defense, ball pressure starts everything. Pressure means pressure. "Head on the ball", "don't back down", and "crawl up into them" are just a few exhortations. Your job is to turn the dribbler in the backcourt and "force to tape" in the forecourt. If you can't touch the dribbler, you're too far away. If the dribbler picks up the ball, immediately surround the pivot foot to reduce ballhandler options, exposing her to more pivot, pass, and time pressure.
I spent much of my junior year chasing around this jackrabbit of a teammate in practice who could really dribble. Think Rocky and the chicken. But it paid off the next season.
Closeouts? Recover into ball pressure. Trapping? Intensify ball pressure.
Ball pressure allows more defensive flexibility. You can choose a more "Pack line" defensive approach with more help, denial off the ball, or front the post - but without ball pressure nothing will work. Great zone defense looks like great man defense on the ball. Great man defense uses some zone principles away from the ball.
It begins with mindset. "I pressure the ball." Avoid negative thinking. "What happens if I get beaten off the dribble or after a pass?" You have to 'trust the protection', but embrace that you are responsible for the coverage. If you get beat, you're a cornerback; turn and sprint. "Basketball is not a running game, it is a sprinting game."
What about fouling? Aggressive play demands intelligent play. Move your feet; show your hands. Most fouls reflect poor technique rather than aggression.
Urge players to "be hard to play against." You must play fearless defense. Tell them, "you can always improve" and "ask yourself what am I doing to improve?" Defense starts with ball pressure.