Some casual hints:
You can get a good feel for other distros by using vmware rather than installing the distro, but if you do decide to split up your drive:
If you install each using ext3, then you'll only need one swap partition.
You can have four 'primary' partitions per drive, so the fourth one will be an extended partition, which you can split up as you want. I wouldn't bother with more than 20 gig per distro.
RPM-based distros will ignore other linux distros when configuring grub, so it's best to install a Debian-based distro on the mbr. However, the current version of Ubuntu neglects to put a root partition number in it's menu.lst, so be sure to add a 'root=(hdx,y)" line when installing it.
Each distro will want to overwrite the mbr, so it's a good idea to print off each menu.lst entry to make sure it's available later on, if needed.
Since it's senseless to keep overwriting the mbr, I put the rest of the distro's grub on the root partition, rather than the mbr. Then, I use a configfile entry to boot the distro:
That works well with distros that change kernels, since the boot's passed from your main mbr's to the grub on that distro, which will have an updated menu.lst after the change. But, keep in mind Ubuntu's weird menu.lst - probably have to add another 'root=' for that new kernel.
LILO and Grub don't always play 'nice'. For slackware-based distros which use LILO, copy down the boot information and type it into your grub's menu.lst manually. Configfiles won't work here.
And, that's about all I can think of at the moment.