Keep a copy of your boot sector packed away for a rainy day
Installing a boot loader (such as LILO) can be trickier than one might like. Particularly when using IDE hardware, it's easy to get yourself into trouble and work a system into a state that makes it impossible to boot without a rescue disk.
One common mistake when using IDE hardware is to install a kernel on a partition that extends beyond the 1024th cylinder. The symptom is very strange, because a machine will boot fine at first, but installing a kernel later (after the machine has been used for some time) makes LILO throw an error and will refuse to come up on the next boot. This can be a very confusing symptom, since "it used to work." Most likely, the kernel that was installed when the system was built the first time happens to reside on a space on disk before cylinder 1024. After the system software and user data are installed, the disk begins to fill up. When the disk contains about 500 MB (or 1GB on some BIOS) of data, any new kernels will necessarily lie (at least in part) beyond cylinder 1024 — which is inaccessible to the BIOS at boot time.
Modern versions of LILO will refuse to install such a kernel, but some older versions simply throw a warning, and install anyway. One way to deal with this is to make a small (say, 10 MB) partition at the beginning of your install process (as /dev/hda1, the first partition on the disk), and mount it under /boot. Now when you install ...