I found this step-by-step on a macrumors forum.
(http://forums.macrumors.com/showthread.php?t=774096)
I havent tried it yet, but it should work.
1. First check your existing partition. You want to be sure the drive has enough "empty space" with which to create a _new_ partition to install SL onto.
2. Assuming there is enough space (I would reckon the minimum would be about 30gig, but I have SL running smoothly at the moment on a partition that's only about 17gig!), launch Disk Utility.
3. In DU's left window, click the "topmost" representation of your internal drive.
4. Click "partition". You should get the window with the rectangular depiction of your disk.
5. Choose "2 partitions". DU should now display 2 volumes.
5. Grab the "dividing line" and drag it to size the soon-to-be-created partition to the size you wish. I will _guess_ that DU "reserves" a certain amount of space for the files of the existing (single) partition, plus some "free space" on top of that. In other words, you can't create a new partition that would take up more free space than is currently on your drive.
6. When you have the partition sized where you wish, click "Apply" down below.
7. It will take a few minutes for DU to do its thing. I'm guessing that before DU actually "divides" the drive's directory, it does some adjusting and moving (if necessary) of the files already on the drive. This "cleans up" the physical sectors where the new partition will go.
8. When done, you should now have _two_ volumes that appear on your desktop. One is the "old drive". The other will be the new one, as-yet with no files on it.
9. I'd suggest a "test boot" from the old volume, just to make sure it starts up, that everything looks right to you, etc., before going further.
10. When you're confident the old volume is fine, boot from the Snow Leopard DVD and choose to install to the NEW [empty] volume.
11. When done, you should have TWO bootable volumes - your old one, and the new SL. You can choose which to boot from using the Startup Disk preference pane. Or, even easier - hold down the option key when you start up and keep holding it down until the Startup Manager appears. You'll see what to do!
12. When the time comes that you feel confident you no longer need your old System, you can either do a SL install "over" the old system (on the old partition), or - another way to do it is to be sure that your new SL volume is the way you like it, and "clone it over" to the older partition using SuperDuper.
13. When all is said and done, I'd recommend keeping that second bootable partition around. It's ALWAYS preferable to have a second volume close-at-hand that you can start up from in an emergency!
Hope this helps
Robert
