Today, form is generally the largest point category for judging dahlia blooms. it's followed (in descending order) by color, substance, stem, foliage, condition, and staging and bloom condition.
Dahlia shows feature categories for single blooms, multiple blooms of one variety (usually triples, all of which should be uniform in size and color), and collections of five or more blooms of one kind. Entrants are expected to name the varieties of their entries, while in county fairs it's often sufficient to enter blooms by category or just labeled "dahlia." (Note: to identify your dahlias correctly, be sure when you plant your dahlia tubers in the spring to label the site with the name or variety number.)
In the display vase or bottle, single blooms should be of a height proportionate to the size of the bloom. Side buds should be removed well before the flower is cut (this is called disbudding) to display an adequate stem with leaves visible above the mouth of the container.
It's important to read the show schedule or fair premium book to find out such things as whether you're expected to provide your own containers and, if so, what size they are to be. You should also be able to place them in the correct dahlia category (by size, or type, or color, as required).
Flower arrangements at dahlia shows, as opposed to the categories in general flower shows and fairs, are often limited to bouquets, baskets, and large-scale standing arrangements. There may also be dried-dahlia and corsage categories.
When showing dahlias, it's important to cut the blooms after they've opened fully -- but only just after opening, so that they retain their fine form through the duration of the exhibition. To help them last well, condition them by standing them in deep, slightly warm water that contains a supplement to extend blossom life.
Showing your dahlias is fun. It also gives you a chance to compare your flowers with those of other growers. And of course, it introduces you to new varieties of dahlias you might want to add to your garden next year.