As we all know, after the first few weeks of practice the intensity level can really drop, especially on defense. I learned this drill from a former Duke assistant while I was on his Ivy League staff. The key to it all is to award points for defensive play, not made baskets on offense. The offense has to score a bucket to win the 'privilege' to play defense.
Make teams of 4 and begin as you would shell drill. Explain the scoring - at first the kids can't imagine scoring points on defense! Award 1 point for a defensive rebound, 2 points for a turnover or steal, and 3 points for taking a charge. If the defense stops the offense, the offense goes off. If the offense scores, they've just earned the privilege to play defense. The next group always comes in on offense. Establish a score to play to (I like it low, say 10 or 12 points).
Minimum 8, ideally 12 (4 on 4 on 4)
We like to run so I add transition once the kids understand the drill (about 2 or 3 days). For example, on a defensive board or a turnover that doesn't cause a dead ball (travel, out of bounds, taking a charge, etc.), the defense can break for 1 more point by scoring in transition.
If you believe in practicing 1st-string, 2nd-string, etc., you may want to handicap the points like the odds at a race track. That third group will probably hate this drill if you don't.
Manipulate the points awarded to fit whatever needs your team has. If no one will take a charge, weight it up to 5 points. We sometimes lunge too much into passing lanes and get caught playing 5-on-4 so I'll make some steals worth just 1 point but forcing a turnover with good ball pressure might be worth 3 points.
You have to 'officiate' the drill seriously. Once they know score is being kept, every close call is arguable.
If you need to you can work in scouting reports. Let's say your next opponent shoots the three like it's going out of style. If the offense makes a three, not only do they get to play defense but the defense is penalized a point. The same could be done for points in the paint or a specific scorer who has to be shut down.
Encourage your groups to plan offensively. You'll find that your kids are probably better coaches than you thought. And you won't believe how ball screens start popping into your offense! They quickly recognize what a tough decision it forces the defense into.
Keep it fun and the kids will respond. Today's kids respond to keeping score. On days like preseason Saturday mornings and Christmas break practices when games aren't just around the corner, we'll go 45 minutes or more with this drill. And, even the, the kids beg to play it just one more time.
Team defense and, more importantly, a way to keep practice intensity high
This basketball drill is licensed under a Creative Commons License. This drill is from Pål Degerstrøm at www.degerstrom.com/basketball.