You can only do so much with a shield. Shields don't move or resist. If you want to improve your tackling I would keep these steps in mind. (Assuming you are playing a drifting defense where you alight on the ball carrier's inside hip) 1) Alight Correctly and shrink the distance - You may want to stay about a yard inside. 2) Track down the runner properly - Always stay inside until you can make contact. When tracking down, always keep the eyes on the hips. If you cross the ball carriers hips, you just gave them an inside alley to cutback into and will probably miss the tackle. When a ball carrier makes sidesteps and other moves, the hips have the least movement and always lead you to the tackle. 3) Make good contact keeping the eyes on the hips.- Depending on the ball carrier, you may want to tackle lower or higher. If you keep the eyes on the hips: you can see what you hit, you won't put your head down, or get fooled by a last second move. 4)Run you feet as hard as you can upon contact- Your power comes from your feet, use them instantly upon contact. You want to run your feet harder than the ball carrier. If you have trouble completing the tackle, you may need to slide your arms up the body and fall to the ground pulling the runner with you. You can also stop a runner held up by using your arms to pull their knees together, the ball carrier usually loses strength and balance in that situation.