We have a Mac running Snow Leopard that is not ready to upgrade to Mac OS X Lion. In order to test out Lion, we create a second volume on the hard disk and install Lion into this new volume. Once the Lion setup is completed and upon reboot, Mac OS X Lion becomes the default OS that the system boots up.
To select to boot from a different volume, hold down the Option key while the Mac starts up. The boot volume menu appears on the screen showing all bootable volume on the Mac. Select the volume that you want to boot from, and click on the up arrow to start booting.
The above picture shows the boot screen on our Mac. The Macintosh HD holds the Snow Leopard, Lion HD volume is the newly installed volume for Mac OS X Lion, and the Recovery HD is created as part of Mac OS X Lion install for recovery purpose.
You can set a volume to be the default boot volume from the same screen. First select the volume to set as default boot volume. Hold down the Option key. The up arrow turns into a circular arrow. Click on the circular arrow to start booting. This will setup the selected volume to be the default boot volume.