I carry a few large zip-lock freezer bags and a 1/4 roll of hockey tape. When the rain is about to start get a bag out, cut a short line down part the seam in the bottom (half the diameter of the lens), and then stretch the plastic around the lens shade. Tape the stretched plastic to the lens shade. Then cut two small holes for the neck strap to pass through and tape around the points where the strap passes through the plastic. This quick set up lets you shoot with the bag completely covering the camera or still allows you to open or close the back "zipper" to see through the viewfinder clearly, press buttons, and review images. It does not however allow you to easily switch lenses (but you can by removing the tape around the lens shade, rolling the bag back, switching lenses and then reapplying the tape to the new lens/lens shade, but watch you don't get water drops inside the camera).
I use this set up in rainy conditions and have only ever had one problem. I shoot with two 5D's at once and two different lenses. Once while working in a downpour for hours I failed to close the zipper on the back of one camera while it was hanging by my side and I was shooting with the second camera. The rain was falling heavily onto the back of the camera and soaked in around the buttons and made the camera act erratically/malfunction. I left it over night with two fans running close by, all doors, flaps, etc open, and I said a prayer of faith. In the morning it was back to its good old self, as if nothing had ever happened. So in downpours, make sure you close the zipper to complete the "weather sealing".
Also, using a lens shade is essential in the rain to protect the front element from getting covered in rain drops. The deeper the shade the better. It also helps to have extra UV filters handy to switch out and in as rain does accumulate on the lens (UV filter) and you don't have time to clean it until later.
That's how I do it.