just remember that you've gotta be the most diverse player, in that you're the first real defense, and you're also the start of the offense, so you can't let yourself be up in the offense all the time, and have the other team overrun your defense. like Dreath said, try to stay on your feet unless you absolutely must go to ground and you know you'll make the tackle.
you also have to be aware of the defense behind you. if your opponents attack and your team semi-clears the ball, and they get it back and your defense is out of position, you've gotta contain their offense and make sure that they can't attack until your defense is in shape again: therefore you need to have your head on a swivel and be able to see when your defense is not set. knowing when to contain and when to be aggressive is one of the biggest parts of your position. if your team is down late in the game and the other team is just trying to play possession, you need to get the ball back. if you're holding onto a lead, you need to make sure that they don't get the opportunity to score, even if it means making them keep possession. in that situation it's better for them to pass the ball back to a defender or midfielder rather than you trying to win the ball, because if you miss then you leave your defense outnumbered.
above all this, never give up or jump out of tackles or let your guard down. you've gotta be the toughest, and smartest player on the pitch.
hope this helped