The springtime weather in New Orleans can range from pleasantly warm to uncomfortably hot, so you'll want to dress in cool, unrestrictive clothing.
Lightweight cotton is never a bad idea.
Therefore most people consider sunglasses to be essential, not to mention sunscreen or sunblock. A hat with a wide brim or visor is also a good idea. Wear comfortable shoes, as you may be doing a lot of walking. Also, several stages on the infield and in Heritage Square are under tents that offer relief from the sun.