UPDATE: Looks like a one-liner was added to apt-get a few years back that simplifies the older and more manual way which I documented below. This also affects more than just the old kernels so use with caution:
apt-get autoremove --purge
After a number of kernel upgrades the file system gets filled up. Here is one way to clean up.
As root –
Display a list of installed kernel packages:
dpkg -l | grep linux-image
Display the running kernel (you don’t want to really remove this one do you?):
uname -a
After careful observation, remove the packages:
apt-get purge linux-image-3.2.0-23-generic, linux-image-3.2.0-24-generic
As a side, I tend to leave one additional kernel installed as a backup, unless there is a serious security/bug issue.