Dresses/Skirts

Perhaps the best kind of '40s dress or skirt to dance in is the "twirly" kind, shown above left--a pleated fabric that billows out nicely during spins. The example on the right does the same, but not quite as well, because of the smaller amount of fabric, and hence a lighter weight. However, if the material is slightly thicker but still very flexible, and there is enough of it to not be tight at the bottom hem, a nice twirly effect can still be obtained.
Knee-length dresses were popular in the '40s, and would be fine to wear dancing as long as the bottom hem is not so tight as to restrict movement. Two important things to look for to help identify 1940s style are shoulder pads and fitted waist.
In addition to dresses, skirt-and-blouse combos were common. The addition of a jacket lends an air of formality, if that is desired.