This is one easy stuff if you have a good camera in hand. First of all I will show you some waterfall which are best captured:
You will not be always lucky enough to get a waterfall which is multi-directional or different streams are coming. But what you appreciate in these photos are the sharpness and the streaming of water.
Now, this is what you will get when you shot waterfall in automatic mode or with normal setting.
Now, change it into low ISO, 100 and shutter speed of 1/10 or 1/8, you will get something like this:
I made it black and white just for fun. But we can easily differentiate the water stream. Now not only this, as it go for low ISO, hand held photo can be tough to take. What can be done for that - there are 2-3 ways to overcome that.
1. Use tripod if you have.
2. Use IS lens, Image stabilizer lens manage small shake.
3. I don't have both and so, I always try to reach near the subject and try using small-lightweight lens rather than heavy lens.
Just tried to write for new photographers like me.
Some good hand held work done by Srikanth: