(via www.timburgess.net)
The grub bootloader is particularly useful, especially when upgrading and testing new kernels. I recently found a way to modifiy the default boot config to make it useful when you wish to remotely upgrade a kernel and boot into it once only without being at the console to change the default back.
Booting once only with grub
# grub
grub>savedefault --default=0 --once
If new kernel boots up without error, edit grub.conf to make the new kernel as the default.
grub savedefault
echo "savedefault --default=0 --once" | grub --batch