For system installed kernels (installed via apt-get):
1) Make a list of installed kernels:
$ dpkg --get-selections | grep linux-image
2) Remove the kernel:
$ sudo apt-get purge <kernel_to_remove>
For manually installed kernels:
Since I'm not using rpm based distro, I haven't searched about it yet.
The most important thing:
1) After all cleaning up the kernels, you must update the boot-loader before rebooting the system. i.e. $ sudo update-grub
in case of grub boot-loader. Not sure how to update LILO.
2) Do not do these operations on current kernel you've booted into. If you want to remove a specific kernel, boot into any other kernel than that.
If all that you wanted to do was to reduce the size of your boot menu, then removing the /boot/vmlinuz*KERNEL-VERSION* file(s) would be sufficient. I usually monitor my Linux updates and when it wants to install a new kernel, I rename the prior kernel (I usually have 2 kernels in by boot menu, the current one I am running and the prior kernel) /boot/vmlinuzXXX file so that when the grub update runs only the current and new kernel get added to the boot menu. By renaming, all of the old kernels are still there and available, and then don't take up that much space, and besides disk is still relatively cheap.