I use this: helmet rain cover.

It has a nice flap down the back to stop rain going down your back; it has a nice big brim to stop rain going in your eyes/on your glasses; has a velcro flap in the back for rear helmet lights; and it comes in bright yellow with reflectors.

I run a helmet light on the front inside the cover and I look like a giant blinking lemon. Who wouldn't slow down when they saw a giant blinking lemon?