So I've been playing a crapload of Left 4 Dead (PC) these last couple of days. I've won the demo on Expert 4 or 5 times now, and I've found a number of things that work. Here's my first tip.

When you know a horde rush is coming (usually the musical cue), get somewhere narrow, like a hallway, stairway, or subway car. Have each end player crouch facing their respective end, and simply use the melee attack repeatedly. The two middle players face towards their respective ends as well, and shoot the stunned horde. Do it well, and the zombies may not even be able to touch you.

Do this and you might actually survive a horde rush on expert without a pipe bomb.