Always stand goal-side of the opponent. It's sometimes natural to follow your opponent closely thats why its safer to keep a distance.
Always stand side-facing ur opponent, and not head on. This allows you to maneuver left or right quickly especially when your marking a really good dribbler.
Always overlooked: Aggression
Don't be shy going for tackles or even blocks, even if you know the person shooting has Gerrard-resque power. It's your job as a defender. When going for the tackle, position the in-side or out-side of ur foot between the ball and your opponent, and follow up with your thighs and ankles 'between' the dribbler and the ball. That way, you have the ball, and because of the position of your legs, your opponent wouldn't have ample space to counter possess since you're a step ahead at shielding the ball with part of your body. Tips i learned from how Roy Keane tackles.

Technically, when you challenge for the ball, standing or sliding, it's called tackling.
Obviously sliding is prohibited, I'm not sure about the butt part. Like, what If i did a legitimately good standing tackle, but somehow I lose balance and fall on my butt? I think logic dictates it's not a foul since we know first hand the tackler wasn't going in for the slide, but I guess it's still up to the referee to decide.
Hopefully someone could clarify on this.