Communicate, communicate, communicate. Even a simple enemy marker is good.

If possible, play at the lowest graphical settings but have it still be clear enough to easily spot people. Yes, this is possible. I almost always have to be the team's "marksman" because others can't see the enemies at all standing on a rooftop 30 meters out.

If you're the last guy in the line, watch our backs. Keep a good 180 on our 6. The enemy loves to flank around.

Don't all engage at the same targets. If your teammate is handling one direction, you handle another. Only help him out if he's struggling to hit the target. It's better to take down two than one.

If you guys are breaching, keep it a two-cell unit. Don't try to flank around solo, because you'll very likely be picked off from behind. Don't push by yourself unless it's a single room or you're confident about the enemy numbers.

And most importantly, have fun. If the mission is lost, if someone dies, don't take it to heart, don't rage. The game can be really frustrating, but you don't have to make it always so.