1) Do not buy beads unless you want something fancy to wear. You will get tons of beads or 'throws' as the natives call them.
2) Do not wear white. It will not stay that way long. The streets are dirty with discarded beads, plastic bags, beer cups, etc. Most wear jeans and old clothing.
3) Be prepared for cold weather. New Orleans is south of the Mason-Dixon line but Mardi Gras is usually in February. Dress in layers as the wind off the river can be chilly.
4) Bring a folding chair if you cannot stand for hours on end.
5) Have a hotel within walking distance of the parade route. The parades do not go through the French Quarter. Hotels along St. Charles Ave have viewing stands for their patrons.
6) Be adventurous and try new foods, New Orleans has some incredible dining experiences like: Beignets (fried dough) at Café du Monde, Muffaletta sandwiches at Central Grocery Store, Cajun cooking at Mulates, Hurricanes at Pat O'Briens, great beer and jazz at Crescent City Brewhouse. I've tried them all and want to go back for more.
7) Be prepared to enjoy southern hospitality. Everyone is there for a good time and want you to enjoy their special party.
8) Most recommend visiting New Orleans before seeing it during Mardi Gras.
9) Ash Wednesday does not end the partying in New Orleans. St. Patrick's day, March 17th, is the next big party as many Irish settled the area in the 1700s.
10) Bring an extra suitcase as you will return with many souvenirs.