1. Try to shoot on cloudy days especially if the waterfall is facing north causing you to face the sun, this will give a reflection of the back of your camer causing small prism shapes in the photo. It will also leave a haze.
2. This is a must, bring a tripod when shooting at slower shutter speeds without a tripod you will get a blur from camera shake.
3. a polrizer lens, this will take the glare off the water your shooting a allowing you to see the rock below the water.
4. This next one is by pure preference and depending on the waterfall some photographers like to shoot a waterfall from the bottom. I like to shoot all over depending on the waterfall I have found that if you can find a spot about half way between the top and the bottom try from there I have done this a few times and I like it.