Digital Photography School has a very nice post on 4 tips for shooting drop dead gorgeous waterfalls. Most people think that all you need is a tripod and some ND filters, but Eric Leslie explains in more details some other important tips, from what time of the day to shoot to what kind of waterfalls require what length of exposure time.
The two tips I like the most are about reading the waterfall to determine how long of a shutter speed you need and about taking a plate at a fast shutter speed in order to freeze the motion of the leaves in the winds and blending that back into the longer exposure shot of the waterfall in post. This can also help if you want to do a HDR composite of the entire scenery and still want to do a beautiful waterfall at a long shutter speed.
Be sure to read the rest of the article on Waterfall shooting.