Share your waterfall photography tips here. A few to start:
Use a circular polarizing filter to cut down on the reflections on wet rock (you can buy circular polarizers here
Use a tripod for those long exposures.
Shoot on overcast days to minimize strong shadow-to-highlight contrast.
Under-expose to keep the water highlights from blowing out. Shooting in RAW helps you recover your shadows if they tend to under-expose because of this.
Watch your white balance... often cameras will create blue water if you meter right on the water.
An exposure time of 1/6 sec is perfect for water with a great sense of volatility.
Tall waterfalls, like Taughannock, or waterfalls with a lot of swirling suds in the whirlpool may need exposures of more than 2 seconds to get the silky effect looking right.
Large, heavy waterfalls, such as Niagara Falls, look best when captured at high shutter speeds of 1/100 or more.
Try to include more non-moving objects (stones, trees, people) in the frame than there is moving water. Often the surroundings are more interesting than the moving water itself.
A white balance of 5200 to 5400k is a nice goal for frothy water on a slightly overcast day.