One of the little tricks I learned while conducting webinars, training, or presenting, is to always have cough drops (throat lozenges) with me.
I generally take one about 10-15 minutes before I start and it seems to help calm my throat and clears out my sinuses. This in turn helps me speak better. It also, of course, helps with coughing which happens to me when I speak too much and do not drink enough water.
Try it out yourself.