The first thing to remember if you want to prevent the other team from scoring, is tackling. Tackling is, bar none, the most important skill in football, and the one that can be taught the most easily if the coaches push hard enough.
Insist upon proper tackling. Don't ask, demand.
I use a four point tackling technique. Deviation from, or improper demonstration of any of the four points will get the offending player sent to remedial tackling, where they work with a coach until they have tackled a dummy correctly five times. This coach should be well trained in your tackling procedure and standards, and should consider this his primary job for the season. Poor tackling in scrimmages and drills can also cause the player to lose their position on the depth chart.
Please note that by using these techniques, you agree to indemnify and hold Coach Wade and Football For Youth not responsible for any and all injuries that may occur. Football is a dangerous sport, and no amount of equipment or preparation can completely eliminate all risks.
Always block and tackle with your head UP!
My four points are as follows:
The tackler is to drive his shoulder into the ball carrier's thigh pad, and nowhere else! His head should be across the runner's body and up. His hips should be under him. There should be no bend or arch to the tackler's back.
The tackler should fling his arms around the ball carrier's legs and pull the runner towards him with a sharp motion. His fingers should be grabbing cloth wherever possible.
The tackler should lift the ball carrier slightly (roughly an inch or so). This serves three purposes: it keeps the tackler on his feet, preventing dives and misses; it breaks the runner's balance and contact with the ground, and it makes the next point easier.
With short choppy steps the tackler should power the ball carrier across, rather than back along, his initial motion.
Most of the game film I've seen and books I've read suggests that poor tackling is responsible for about 75% of offensive production in youth football, and about 85% of the scoring. If you eliminate that poor tackling, you have a better chance of preventing first downs, and a better chance of keeping the other team out of your end zone.
Field position is also the name of the game in youth football. I had an outstanding defense in 1999, and an average offense. Our great defense got us the ball inside our opponent's 30 yard line at least twice in every game, and that led to at least one score per game, which in turn led to an undefeated season. Proper tackling can help you on both sides of the ball.
I use one major tackling drill. Put two lines at right angles. One line is ball carriers and the other is tacklers. At the snap or coaches motion (not a whistle. You want the kids to stop on a whistle. Start them with ball movement.) The ball carrier jogs at 3/4 speed in a straight line. The tackler moves as fast as he needs to to make the tackle, following the above four points. Alternate shoulders for every repetition and change angles at least daily. Do this drill every single day! It only takes ten minutes a day to perfect your tackling if you don't accept shoddy effort. By contrast, it only takes one missed tackle to knock you out of the playoffs. Tackling should be a full team skill, not just something your defense works on. Generally, our tackling drill is the first one we do after our five minutes of warm ups. Another series of tackling drills is available here.
Never, ever do full speed tackling drills. Drills are where you perfect technique. Do your full speed tackling during your scrimmages, and scrimmage infrequently (about once a week or even less should be fine). If you drill at full speed, I can virtually guarantee that you'll be calling an ambulance at least once during the season. (We had one relatively minor, although scary, injury on my team in 1999... guess what drill my assistant had the kids doing. By contrast, we had zero injuries during my 3/4 and 1/2 speed drills. The rest of the league had six injuries that ended seasons. Iím not positive, but I believe most of them occurred during full speed tackling drills.)
I would be very remiss if I attempted to take complete credit for this tackling procedure. Quite a bit of it comes from the great books Coaching Youth Football and Coaching Youth Football: Defense by John T. Reed. I recommend greatly that every youth coach read and reread these books. There are an awful lot of high school coaches out there that would also benefit greatly from the material Coach Reed has to offer.