@GuySoft answer needs to be updated based on Raspian Jessie from this answer:
DO NOT MODIFY /etc/network/interfaces
Add the following to the end of /etc/dhcpcd.conf
interface usb0
static ip_address=192.168.42.42/24
static routers=192.168.42.1
static domain_name_servers=8.8.8.8
routers and domain_name_servers line may not be necessary