BarryO Here's a few ideas, off the top of my head. Start with a color theme based around the healthiest, best dahlias you have. The dahlias should be the focal flowers and "stars" of the vase. Use a variety of plant material textures and colors what will compliment the dahlias and color theme, keeping in mind the desired overall "feel" you're aiming for, such as formal or casual. Imagine the arrangement is for an event, and to help set the theme ( country picnic, wedding reception, baby shower, etc.)
Make sure the plant materials that are part of the arrangement have been "conditioned" so there's no drooping of the materials after creation. Keep the water level topped off so no stems are out of water. Place the completed arrangement in a cool location overnight( I put my flowers outside in the 50 degree F. evenings).
And lastly, have an identical mason jar available. On the day of the judging, place fresh water or flower preservative solution in the extra jar. Carefully transfer the arrangement(rinse off stems if needed) into the fresh water in this "extra" mason jar. Having fresh water in the clear mason jar will contribute to the perceived freshness of the arrangement.
If you have available plant materials, make several similar arrangements and pick the one you think is nicest.