Breathe deeply before you speak by breathing in using the muscles of your diaphragm. Shallow breathing will prevent you from projection and will give the impression that you have a weak voice.
Speak, don't sing, familiar melodies that you know from childhood. Choose melodies that are lyrical and will allow you to practice moving in a rise and fall motion. To do this, match the pitch of each note in the melody without activating your singing voice. This will expand your voices ability to be more expressive by increasing your range. You don't have to speak the melodies in rhythm, take your time to expand your range of pitch.
Learn how to create natural rises and falls in your speech patterns. Ascend in pitch toward the middle of a sentence and descend toward the end of a sentence.
Accent words that are essential and that stand out as important in your speech. This will help to add expressiveness by creating a rhythmic emphasis that brings attention to important words.