What’s your Left 4 Dead advice or tip?

In the Left 4 Dead series, your pistol has unlimited ammunition. If you find that you're running out of ammunition often, you should keep the pistol out most of the time, and only switch to a better weapon when you're facing a horde or a tough opponent. Also, it's usually faster to switch weapons (using Y on the XBox 360 controller) than to reload your current weapon (unless that one's out of ammo, too, in which case...run!)