The proper attire IMHO is: Clean starched jeans/slacks (that fit!), a belt, pressed long sleeved buttoned up shirt (even if it is summer and HOT!), clean boots, groomed hair, and a wonderful attitude. Like ROMAX stated make sure that your shirt color works as an accent with the color of your animal, no matter what the species, and as Showheifer said, "make sure it covers everything".
My children wear the tear away pants over the jeans/slacks and either a tank top or t-shirt so they can put the show shirt on after fitting, but before they enter the arena this way they are always clean and professional looking. If you wear a shirt under the show shirt when in the arena you can take the show shirt off between classes if there is a long time between classes.
We have a couple of friends who are judges and they always tell the showmanship classes, if it is close between exhibitors the one who looks the part will always win and long sleeves will win over short sleeves no matter how hot it is. Consider every class a showmanship class, remember a good showman can make a good animal look even better, but a poor showman can make a good animal look even worse.