I prefer a simple rain cover for my camera, like the one from FotoSharp. This simple waterproof cover can be secured to a tripod-mounted camera and keeps my equipment dry. The cover compresses to fit in my pocket when I'm done using it. I also use a protective filter and lens shade when shooting in inclement weather. It's inevitable that the front of your lens will get wet, and a little water on the front element isn't a problem. But I've seen rain showers soak the front element to the point of creating humidity inside the lens. A simple UV filter attached snugly to the front of your lens offers one more layer of protection.
Lens shades are great for keeping rain and dust from hitting the front of your lens. And if your camera gets blown to the ground, your lens shade might just save your lens from breaking.
Have a polarizer ready to go. If conditions are right, rainbows often form after passing showers. In some places, like Hawaii or Patagonia, they seem like a daily occurrence. A polarizer will help intensify any rainbow you encounter. Just screw one on when you see a rainbow and watch the colors pop. Polarizers also add contrast and saturation to stormy skies, which helps add drama to stormy scenes. I use a Singh-Ray LB Color polarizer, which polarizes the scene and punches up the color. For the best results using a polarizer, shoot at a perpendicular angle to the sun.
Catch lightning. There are two basic approaches to photographing lightning. If you shoot at night, you can use a long exposure like 10 seconds and wait for the lightning to strike. If you have an active storm, capturing a lightning strike isn't too difficult. You'll have to use manual focus since autofocus doesn't work in the dark. Focusing at the infinity mark on your lens will work for distant scenes. If you have close foreground details in your scene like trees, try using a large flashlight to illuminate the trees and help you focus.
The challenge is photographing lightning during the daylight hours. Since it's daylight, your shutter speed will probably be one second or faster. Unless you have Zen-like weather karma, you can't time your exact shutter release with a lightning strike. To solve this problem, you need to use a device that automatically fires your camera when lightning strikes. There are a few to choose from—I use a system that attaches to my hot-shoe with a cable that connects to my camera. Anytime the sensor "sees" lightning, the shutter is released, capturing the bolt. Similar to shooting at night, you need to turn off your autofocus and use manual focus. You'll miss images if the camera is trying to autofocus during a lightning strike.
Add a sun star to the image. After passing thunderstorms, the sun often peeks out from the clouds. In addition to creating rainbows, the sun can be photographed as a sun star, adding another compositional element in the image. Try this technique. First, use a small aperture like ƒ/16 or ƒ/22. This tiny opening helps enhance the sun-ray effect in the image. Next, take off any protective filters you have on your lens. Flare can be a problem when you're aiming at the sun, and front filters are often the main cause.
To determine exposure, meter a middle-tone scene without the sun in the frame. Use this exposure as your starting point. I often underexpose my image slightly so the sun and rays aren't washed out in the final shot. Different lenses will create different-looking sun stars. Fixed-focal-length lenses generally produce less flare than zoom lenses, since there's less lens glass to cause flare. My favorite lens to create sun stars is my 24-120mm ƒ/4 lens. This lens creates multi-rayed sun stars that really add drama to a stormy scene. My fisheye lens produces the least amount of flare when shooting sun stars.