This is from a Mount Holyoke website:
Prof. Straw, Department of History
HISTORY AND PUBLIC SPEAKING
4. Time and practice. Practice so that you can deliver your presentation fluidly and confidently. Enunciate words carefully: this means no Larchmont Lockjaw, or lazy mush-mouths. Move your lips, tongue, and mouth. Remember Demosthenes mastered his language by speaking with a mouthful of pebbles, and Liza Doolittle had a professional coach. Practice difficult constructions so that you do not stumble through them.
http://www.mtholyoke.edu/courses/cstraw/OralReports/speakingtips.html