This is from a match I played on friday and it shows the most common misstake that beginners make when on defence. I know I allways nag about this but you should never go for the body if you're not certain that you'll connect the tackle. Better to concentrate on small moves and being in the right pos with your D's like I've written in previous posts:

Keep your defenders in the slot

Discipline

This video also shows how much can be accomplished just by being positioned right:





With that said I must admit that I tend to be too passive when defending. Well timed tackles in the slot is very effective but like I stated before - you need to know what you're doing.