What if you're envisioning a calming, peaceful, and blurred-out waterfall, but all you're getting is a stilted stream? If you want to really capture motion blur, make sure you leave your shutter open long enough. And before you do, put your camera on a tripod. When that's not an option, set it down on something flat and use your automatic timer to get the photo without jiggling it. Then dial your shutter speed to 1/2 sec or more.