Well when the group of us really played L4D1, we didn't really plan anything. We just talked about random shit, and occasionally plotted things. Normally, unless you're facing real good people, all you need to do is take advantage of a teammate's actions. Your boomer friend just got all but one survivor? Smoker him into a hallway. A dumbass hunter random-jumps on a group of survivors? Take the opportunity to boomer the whole group while they're trying to get the hunter. Basically:
A) Play with your friends
B) If not, do what I said above. You may not win, but you'll do better. Because even if your teammates don't have mics, all you need to do is react to what they do.
EDIT: As for L4D2, I didn't like it as much, so I don't know as much about it as the others would :P