Comfortable Shoes! Forget about fashion or looking good. Wear what is comfortable. You will miss out on a lot if you can't cover a lot of ground.
It might rain, but I never pack an umbrella or raincoat. You can pop into any store and get both cheaply.
If you plan to go to Bourbon Street (and you should, at least once in your life) you can bring any kind of outrageous "dress up" items you have. Feather boas, costumes, tiaras, crazy hats, costume jewelry.
Toiletries and Medical Supplies: Sunscreen. Especially if you live somewhere North.
Photo Equipment: BRING YOUR CAMERA! If you normally take pictures on your travels, you will want to take them here. There is so much to see-the buildings, the cemetaries, the courtyards.
Check the weather chanel, or call a hotel in the area you're staying in. Ask the receptionist what the weather has been like. For both night and day. Make sure you bring a jacket for night time and be prepared for both cool days and humid days. One can't tell in October.
Toiletries and Medical Supplies: If you're looking for aspirin or antacid I would suggest bringing some. Finding a drug store in the French Quarter is not easy.
Photo Equipment: Bring plenty of film, because this city is like none other in the states. Who knows what or who you might see, plus its the best souvenir.
Miscellaneous: Don't forget to buy beads before you go home!Everyone will ask you for them, and it's the easiest way to brighten ones day at home.
Dress light and cool...it's hot and humid. Take a sun hat AND a rain hat! We got caught in a sudden summer storm that lasted about 2 hours. We were soaked through. A light, compact rain slicker with hood is good to carry in a purse. The storms hit suddenly! Also take comfortable shoes...you'll do a lot of walking!
Photo Equipment: The best thing we did was buy throw away cameras. There light, easy to carry, take great pictures...and you don't have to worry about losing them because they are easily replaced.
Miscellaneous: There are so many kinds of food here...and a lot of it is spicy. So if you aren't use to it, better bring some 'Tums' or 'Mylanta'! Ya gotta try the catfish and beans'n rice. MMMmmmmm! (I wonder if they get the catfish from the muddy bayou?)
Photo Equipment: New Orleans will provide you with some great photo opportunities, so be sure to bring your photography gear.