I also begin giving lidless cups around 18mths of age, sometimes earlier/later based on the ability of the children.

One thing I like to do before going to lidless cups is to give sippy cups without the valves (or whatever piece that is used to make the cup "spillproof".) This makes the water/milk just pour out instead of the kids sucking on the spout, and gets them used to drinking more carefully. It also makes for easier clean up