Don't be afraid to experiment! With digital cameras, you can now store an incredible amount of photos. Try it out! Don't be afraid to set up your camera at a stream one day and play around with the effects to see what kind of photos you get. It can be a great way to learn what works, and what doesn't.
Take your pictures quickly. The longer you hesitate, the better the chance that your subject will move away, break their pose, or become tired and stop smiling. Start taking shots as quickly as you can, and don't worry about getting the camera perfect before the first shot. The faster you shoot and the more photos you take, the better your chances are of getting a good one.