If you're tight on money (and in today's economy who isn't?) you can do what I do.
I can't justify the expense of buying a designed mositure cover, so I just take a gallon-size Ziploc bag and cut an approximately 2,1/2-inch diameter hole about 4-inches from the bottom of the bag (not the top where the zip closure is) and 4-inches from one side.
The size of the hole is slightly smaller than the size of my lenes which helps make a better seal around the lens body.
With the positioning of the hole, I can place the entire camera inside the bag with about 2 or so inches hanging underneath the camera and enough side and top room for accessories attached to the camera. The thinness of the bag allows me to operate all the camera controls and view the LCD sceen with ease.
The lens will still get rained on but the camera is totally enclosed. If the weather really starts to get sloppy, I can always zip closed the bottom of the bag. If the weather gets even worse, I go inside.
This isn't perfect, but it is very, very inexpensive and effective.