For me, the largest part of the battle is the set-up and the approach. Getting into an ideal position quickly and with purpose gives you the initiative. Arriving when the attacker's already moving forward, and you're playing catch-up.
Squaring up to one's opponent with weight forward means the first step is forward. I see too many players with hips turned inward - 'ruck inspecting'. When the ball comes out, the first step is to square up, and then they can go forward. Initiative lost. It might be much to some, but I keep my inside foot forward to prevent me from turning my whole body inward, instead using just my head to look at the ruck.
The approach allows the defender to close down and make the tackle on his terms. I favour long and aggressive steps forward to close down the space quickly and move the tackle line back. Coordination among the three players in front of the ball (at least) is vital so no easy gaps are given. As the defender gets closer, the steps have to become shorter to be ready for sudden changes in the attacker's movement. I urge players to keep moving to the ball and not get caught flat footed or even 'break it down' by stopping and chopping the feet in place - both are susceptible to being stepped and/or run over. That final step before contact is usually the inside foot in between the legs of the attacker, with the outside foot coming forward as contact is made. It's not always thus, but it's a powerful position to strive for.
From there, all the safety and technical stuff comes in - sighting the target, dropping the butt, head to the side, shoulder contact, arm wrap, take down (in whichever way is preferred/necessary). But when I see people miss a tackle, most often it's because their initial alignment and/or approach was insufficient, not as much the specific technical elements.
That said, I still see a lot of pros - even - getting their head in front of the torso or down around the knees going for a chop tackle, and a few have been knocked out because of it. Huget on Ranger is a bad one that can be seen on youtube. My teams don't do a lot of full contact work at all, but we work on the alignment and approach bit a lot, ending with a little shoulder bump to the midsection before backpedalling and repeating on the other shoulder x 10. It's one of the few things I 'drill' in rugby as I believe the safety and subtle technical bits have to become automatic and consistent.
