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 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.
The first thing to do when stepping into the batter's box is to find the proper place to place your feet. Make sure you are not too close to or too far from the plate. Touch your bat across the plate to find if you can cover an outside pitch. Your bat should touch the same place each time you step into the box.
With short choppy steps the tackler should power the ball carrier across, rather than back along, his initial motion.
Effective Tackling Drill
Make two lines at right angles. The first line is the ball carriers and the other is the 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. Drills are where you perfect technique. Do your full speed tackling during your scrimmages or when your practicing full defense and offense