New York Comic Con is criowded but not ridiclously crowded.
Bring:
If not in costume, dress for comfort, especially wear comfy shoes. You'll be on your feet a lot and the floors in most convention centers are cement.
Snacks. Food cost a fortune at these places. Bring nuts, cookies, chips, raisins, etc. Nuts can be dumped in ziplocs or generic plastic bags for easier storage.
A water bottle with a removable cap, not one of the pop-up things. You can pay $4-$5 for a bottle of water so bring your own and refill it at water fountains.
Small poster tube. There are some cool freebies you'll want to snag, so have something to keep them from getting squished. A small poster tube is ideal. You can even tie a string on it and hang it from your belt, across your back, etc., to keep your hands free.
These might sound silly, but you'll thank me later.
Enjoy!