/etc/network/interfaces

auto usb0
iface usb0 inet static
        address 192.168.0.200
        netmask 255.255.255.0
        network 192.168.0.0
        up iptables -A POSTROUTING -t nat -j MASQUERADE -s 192.168.0.0/24 &
        up echo 1 > /proc/sys/net/ipv4/ip_forward &
        up iptables -P FORWARD ACCEPT &
        down iptables -D POSTROUTING -t nat -j MASQUERADE -s 192.168.0.0/24 &

=================================
/etc/udev/rules.d/85-ifupdown.rules

# This file causes network devices to be brought up or down as a result
# of hardware being added or removed, including that which isn't ordinarily
# removable.
# See udev(7) for syntax.

SUBSYSTEM=="net", DRIVERS=="?*", GOTO="net_start"
GOTO="net_end"

LABEL="net_start"

# Bring devices up and down only if they're marked auto.
# Use start-stop-daemon so we don't wait on dhcp
ACTION=="add",          RUN+="/sbin/start-stop-daemon --start --background --pidfile /var/run/network/bogus --startas /sbin/ifup -- --allow auto $env{INTERFACE}"

LABEL="net_end"

ACTION=="remove",       RUN+="/sbin/start-stop-daemon --start --background --pidfile /var/run/network/bogus --startas /sbin/ifdown -- --allow auto $env{INTERFACE}"