One of the beauties of a clear vase is that you do see the stems of the flowers - think Gerber daisy's or tulips. Three or four visable stems adds visual interest. If you would like to hide the stems place marbles, pretty rocks or other decorative non-perishable items at the bottom of the vase.
You can crinkle clear plastic wrap into a shape to fit the inside surface of the vase. Use enough to create an opaque look. Another option is to add small pebbles or marbles that will also help hold up the stems.