Create partition with fdisk
mkfs.ext3 /dev/sdc1 sudo yum install livecd-tools sudo umount /dev/sdc1 sudo livecd-iso-to-disk CentOS-6.4-x86_64-bin-DVD1.iso /dev/sdc1
If not bootable
sudo parted /dev/sdc (parted) toggle 1 boot # 1 is partition number (parted) quit
If you want to just put the netinstall iso on your USB device and install CentOS via URL, do this:
use maxschlepzig's method above to get the netinstall iso on your USB device. That method only works with self-contained ISOs that have all the packages you need for installation.
boot the USB device
at the bootloader screen ("Install or upgrade an existing system" etc.), press TAB to edit boot options
remove the whole stage2=... section.
add "inst.repo=", where the url is something like http://mirror.stanford.edu/mirrors/centos/6.3/os/x86_64
press Enter to boot
it will first try to find preview.img and one other file that doesn't exist, but will eventually timeout and fetch install.img and allow you to go through the netinstall.
see http://wwoods.fedorapeople.org/doc/boot-options.html for Anaconda boot options