It's often easy to anticipate what the child is saying and to begin your answer before the child has finished. It's more positive to allow the child to finish, then pause, and then respond. This is all part of the concept of turn taking. The modelling of talking skills is important. In your own speech, strive for a slower rate and with increased melody and inflection.
Children tend to copy the adult models to which they are exposed, so be a good model.
Reducing verbal competition can be helpful. When children must compete with siblings, parents and other persons for recognition, this can be difficult. The above mentioned ideas for verbal turn-taking, sharing the talking time and slowing down the overall pace of the communication can often prove helpful. It helps to minimize distractions, and other things that compete for the child's attention. For example, it is often difficult to carry on a conversation when the child's attention has been captured by a television, radio or stereo.