Configuring an interface to use DHCP (client), just put this in your /etc/network/interfaces:
auto eth0
iface eth0 inet dhcp
to avoid NetworkManager managing it. Restart NetworkManager to pick up on that:
# service network-manager restart
If you need to run dhclient manually after configuring it yourself using ifconfig or ip commands, you just can and I don't see how upstart is involved here.
For example:
Have it configured manually in /etc/network/interfaces:
auto eth0
iface eth0 inet manual
Then the interface is down by default,
# ifconfig eth0
eth0 Link encap:Ethernet HWaddr b4:b5:2f:xx
BROADCAST MULTICAST MTU:1500 Metric:1
so, bring it up with
# ifconfig eth0 up
# ifconfig eth0
eth0 Link encap:Ethernet HWaddr b4:b5:2f:xx
UP BROADCAST MULTICAST MTU:1500 Metric:1
and start dhclient:
# dhclient eth0
# ifconfig eth0
eth0 Link encap:Ethernet HWaddr b4:b5:2f:xx
inet addr:192.168.0.134 Bcast:192.168.0.255 Mask:255.255.255.0