You can buy all sorts of sophisticated rain covers if you wind up shooting in rainy weather quite a bit (take a look at Think Tank Photo's Hydrophobia® covers, which are awesome, but expensive), but the problem is that if the weather unexpectedly turns bad, chances are you might not have a rain cover with you (if you have a small camera bag, it probably won’t fit). That’s why I keep a package of OP/TECH Rainsleeves in my camera bag at all times. They're not fancy, but they're small enough to always have with you in your camera bag (and they work with lenses up to 18" long). Plus, they're cheap as anything—you can get a pack of two for around $6. I've had to use them before, and they do a pretty decent job for those times you get caught by surprise.