I've been able to more than double the battery life of my netbook by turning off a bunch of services I don't need, and unloading their kernel drivers.
service --status-all to see what's running on your system, and
service <service-name> stop to shut it down.
lsmod to see what kernel modules are loaded, and
rmmod <module> to unload it.
If/when you want to bring things back, easiest way is to simply reboot.
Sometimes you also need to kill daemons or programs that are using the service or driver before they can be turned off. Look at output from
ps aux to see what's running, and
kill -9 <pid> to terminate them.
Services I usually turn off include: Ubuntu One, ssh, apache, databases, avahi, pulseaudio, cups, apparmor, acpi-daemon, bluetooth. Modules I unload: The whole audio stack, usb_storage, webcam drivers, wireless, bluetooth. (Some services like audio don't die easily.)
I've even gone as far as shutting down x (
service gdm stop) and working entirely just from consoles, which let me stretch my netbook battery life to nearly 8 hours.