Here's a pretty good tutorial:
http://home.planet.nl/~elst0093/motub/multboot.html
Generally speaking, what you want to do is create a separate partition for your bootloader (i.e. /boot on it's own partition) to store the bootloader config files. Then only install the bootloader for the first distro. Then each time you install a new distro, you just copy the boot files to the /boot partition and edit the config files.
A separate method that I use regularly is to use a 3rd party bootloader (I use XOSL 1.15) on it's own partition. Then each time you install a linux, just intall the bootloader to the root partition instead of the Master Boot Record (MBR). Then just add the bootable partition to the menu on XOSL and it will then chainload Grub or Lilo.