Friendly fire really can get terribly frustrating. It's not that hart to avoid, though.
If you are in front, duck when you shoot. Just do it every time. If you can't see someone in front of you, assume YOU are the point man, and duck when you shoot.
Stick to walls. It's much easier to slide accidentally into someone's cross hair when you're in the middle of a hallway.
If you are not in front, don't be so Goddamn eager to be. Plenty of FF is totally the fault of the person who gets hit. If someone is in front of you plugging away, don't run ahead of them.
Finally, you do have to, at a point, just trust that your teammates know what they're doing. If you see a zombie rushing a survivor, your first instinct is to blast it, but in a lot of cases, that's going to result in FF. So just call it out.