Below is the basic idea of the Target Disk Mode (TDM) method for downgrading your Mac. Please read the first paragraph of "Will a downgrade work for you?" to see what expectations you can have of this working for your given Mac.

Highly recommend you back up your Lion partition and make a Lion Recovery thumb drive before proceeding so that you can re-use Lion in the future. This Apple knowledge base explains how to make the recovery drive http://support.apple.com/kb/DL1433.

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Preparation

• You need a Mac running Snow Leopard (10.6.x).  This will be your host computer. Although technically, if you have a Mac that runs regular Leopard (e.g. 10.5.8) and it is capable of running Snow Leopard, then you can actually use it as your host computer with this method.  Nothing will get changed on the host.
  
• You need the Mac being downgraded (target computer) and a Firewire 400->400, 400->800, or 800->800 cable -- depending on what kind of Firewire ports your machines have.
• You need a retail/box set of Snow Leopard (it might be possible to use the grey Snow Leo restore disc that came with your host Mac but lower your expectations on the outcome ;)
  
• Download the 10.6.8 Combo Updater v1.1 from the support site and place it on your host desktop.

Let's Do This!

1. The Mac being downgraded from Lion (target system) should be off. Connect your Firewire cable between your target computer and the host.
2. Boot the target Mac you're going to downgrade while holding down the T key until a Firewire logo appears on its screen.
3. Back at your host computer, you will now see the hard drive name of your target system mount on your host desktop (it might only appear on left side of a folder/Finder window if you've got your hard drives hidden from your desktop.)
   
4. Launch Disk Utility from the host system. Be very very very careful here, we're going to repartition your target/downgrade drive and we must be sure of the correct one! You want to find the target computer-you-want-to-downgrade's hard drive name in the list on the left, NOT your host hard drive names. Each drive will show up as a cluster of names (2 or more) with the first line showing the hard drive size & brand-model and the second line showing your regular name for it. When you click on the line with the hard drive size & brand-model, it will give you more information about it down below.  If both of your computer hard drives are named, for example, "Macintosh HD," then look for other hints. Look at their icons in the list (one might be an orange Firewire icon) or read the data down below to see if Connection Bus says Firewire (something like this.)
   
   When you've got the correct target-to-downgrade drive, click on the tab that says Partition. Now you've got to make a decision: do you want to dual boot Lion and Snow Leopard? Do you want 1 large partition with Snow Leopard? You cannot expect to change this later so you have to decide now. If you want to dual boot, hit the (+) symbol and add a partition, set their respective sizes (I wouldn't go less than 10-15GB for the smaller partition). If you want 1 large partition, then click the dropdown that says "Current" and change to "1 Partition." Click the "Revert" button to start over. When you're ready, you click the "Apply" button and it will warn you what drive it's going to modify and what will happen when you proceed. Remember, you are using your host computer to reformat your target drive's partition, so your target is the name that should be showing up when you set about partitioning. Good luck
5. Pop your Mac box set/retail/bootable Snow Leopard disc into your host computer's optical drive. Double-click the installer. Select the target-system-you-want-to-downgrade hard drive volume as the destination. If you are dual booting, then you'll set this to the volume you reserved for Snow Leo. Proceed and let it do its thing: your host system will auto reboot but it will boot off your target system's hard drive. Let it finish installing and then you'll go through set up steps.
6. Then run the 10.6.8 v 1.1 Combo Updater you downloaded earlier. Let it reboot. Then shut down your host computer. After that's shut down, you may shut down your target computer and disconnect the Firewire cable.
7. Turn on your target system (in a two-partition dual-boot system, hold down the option key during boot to select the Snow Leo partition manually). It should boot Snow Leopard 10.6.8. You can run Software Update (Apple menu) right away OR, if you like to be thorough, download/run the 10.6.8 v 1.1 Combo Updater again, reboot, then run Software Update.
8. Note: on your host computer, under System Preferences --> Startup Disk you can make sure your original drive is highlighted and selected as your startup disk.