Every videographer's car and travel kit should be stocked with plastic bags. The 40-gallon size with a thickness of 1 mm is big enough to cover almost any piece of gear you're likely to use but small enough that a few can always be carried folded in your camera bag or a raincoat pocket. Clear plastic bags, which look a lot more professional than your run-of-the-mill green trash bag, can be found at any big-box home supply store.
Spread a bag flat on the ground to prevent grass stains when you're kneeling over the viewfinder for a low angle shot or to keep sand or other grime from attaching itself to your camera's bottom when you set it down on a floor that's not clean enough to eat off of. Place bags over lights on a rainy day to keep them dry; just remember to remove them before you light up the set. The extra strength provided by a double bag lets you use it as a makeshift carry-all to collect and carry cables, accessories, and props when packing up after a shoot.
And most importantly, a big plastic bag is the key to keeping your camera dry when you've no alternative to shooting outdoors in rain and snow. Simply invert the bag and place it so the bottom of the bag is over the top of the camera. Now take a thin strip of gaffer's or electrical tape, fix the plastic tightly to the outside of the lens hood, and use a knife or razor blade to cut away the part actually covering the front element. Repeat this process with the viewfinder and you'll have a camera that is practically waterproof while still providing access to the controls and even allowing tape changes by reaching up under the bag. It's not pretty, but it's quick, cheap, and works better in a downpour than almost any other camera cover out there.
