Any smooth surface will produce reflections, but in the natural world, for me water gives the best results. Wet sand, mud and water surfaces, calm or smooth, all produce varying degrees of reflection. In the urban environment, any glass window, particularly those in high-rise buildings, can produce interesting reflections of sky, clouds or surrounding buildings. Even a small variation in your angle of view can enhance or reduce the desired effect of a reflection, especially when you are working close to your subject, so it is important to investigate various angles of view before you start shooting. For example, the reflection from the office tower in the picture below would have vanished had I moved five metres either way.
One of the fascinating aspects of reflection is that the human eye will accept distorted reflections as long as the source is included in the photograph. Cropping out the source of the reflection may produce an interesting abstract image, but it may also confuse the viewer.
The rising sun reflects from a tower block window onto the surface of the Brisbane River. [135/200 mm, f8]
A dingo hunting on the beach is reflected in a thin sheet of water covering the sand, an effect that makes this picture interesting. [135/600 mm, f8]