Painless multi-booting with multiple UNIX systems
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If you have a desktop, or laptop, or server, and want to boot multiple 
UNIX (or other) systems using GRUB, here are some things to keep in 
mind, let us say if you have one partitioned disk.  Whenever I ignored 
my own advice, it has given me headaches:
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<LI>Have a common /boot directory for all systems.  I have a 1.5TB 
desktop and have given the shared /boot partition 1 gig.  It uses 129 MB 
of it - much of it for the 7 initrd images, then some for the 7 
compressed kernels etc.  I have it as ext3 instead of ext4 due to some 
of the older systems.</li>

<li>Sometimes systems update their grub, or their kernels,
re-run grub setup and installation, and this can mess up your other 
kernels.  It is a good idea to copy relevant menuentry's from 
/boot/grub/grub.cfg to /etc/grub.d/40_custom for *each* of the systems 
that you do system updates on, and which might re-run grub setup.  
Standardizing this file across distributions means running grub setup 
should have a standard behavior, no matter under which system things are 
run.</li>

<li>I share swap across all the systems.  Sharing /home depends - I use 
one system as my primary, and it has my big /home partition - on other 
systems I mount a local /home off of root (/), but don't have much 
there.  There is another directory to consider sharing, which I'll 
mention later.</li>

<LI>Have the Master Boot Record (MBR) be on top of the whole disk, not 
one of its partitions</li>

<li>I have a Ubuntu 10.04 partition which I do not use much, but leave 
there so as to boot from.  If I have problems with the two systems I use 
the most (right now Ubuntu 10.10 and Debian sid), I can go back to that 
old reliable one</li>

<li>I grab a copy of the grub.cfg or /etc/grub.d/40_custom files once in 
a while and put them on another machine.  In case I need to by hand type 
in which kernel I want grub to load, if grub setup has some problem</li>

<li>Remember when partitioning the disk initially that, for many cases, 
a disk can only have four real ("primary") partitions.  I usually 
designate the first one the active partition, and the last one the 
extended partition.  I then sub-divided the last partition into logical 
partitions.</li>

<li>My disk usage of root partitions for each system range from less 
than 4 gigs (Gnewsense, which I unfortunately do not use much) to 10 
gigs (Ubuntu 10.10).  Since I have 1.5TB of space, I give them all 
enough extra space to the point where I don't have to worry or think 
about it.</li>

<li>The final thing to think about is KVM - kernel-based virtual 
machines.  I have Ubuntu 11.04, which is currently (as of November 2010) 
in alpha, in a KVM, and I have other OS's in KVMs as well.  The images 
are in image files, which are located in /var/lib/libvirt/images.  If 
you're going to use KVM's across different OS's, this directory can be 
shared.  The existence of KVM's lessens some of the need to install OS's 
to existing partitions.</li>