A good source of clean water is just as important as food. In fact, you will often attract birds to your table simply at the sight of a glistening container of fresh water.
Many birds prefer to drink and bathe in water that is close to the ground. However, if there are cats and other predators in the area then you should probably move the container at least one metre off the ground, or higher if there is a lot of surrounding vegetation, so that the birds have a better vantage point.
If you are using a metal container, pour in some sand before adding the water to provide a more stable footing for the birds. Similarly, the odd stone or branch placed inside the water will enable them to perch and drink without getting wet.
In addition to baths or containers of water, birds are often attracted to dripping water.
It may sound obvious, but never be tempted to add anti-freeze or glycerine to the bird bath to stop it from freezing in winter as this will almost certainly be poisonous to them.