The first line of defense is to keep the lawn well fed (but not now in the UK), frequently mown, at the right height. That will allow the grass to gradually outcompete the clover. Selective herbicides are available, but you should carefully consider the effect on the environment and the local children and pets before using them. If you do, be sure to mix the herbicide with a surfactant, because clover leaves are very good at letting liquids roll right off of them!