Hello and welcome to my User Tip
For some reason or another you need to revert your Mac back to OS X 10.6, I will show you how to do this.
Sorry there is no easy way to restore Snow Leopard in a manner as easy as it was upgrading to Snow Leopard to Lion.
Roll up your sleeves and get your thinking cap on, I've written some very detailed instructions you need to fully understand, if not, then don't bother winging it.
If your Mac is Late 2011 or later issue, then this User Tip has no further use for you, sorry.
If your Early 2011 Mac came with 10.6 or 10.7, then you should try to find your/a copy of the 10.6.4+ machine specific disks for that model of Mac. Call Apple for replacement disks, there will be a fee of course.
If your Early 2011 Mac came with 10.7 and you can't get the 10.6.4+ machine specific install disks, then you need to follow the steps outlined in this thread and create the 10.6.8 disk etc...
You will need to have a Early 2011 Mac or earlier that originally came with 10.5 or 10.6, or upgraded to 10.6 via the 10.6.3 Retail disk, (or have the 10.6.8 combination disk you created or another Mac via Firewire Target Disk Mode) to follow along further here.
10.7 preparation before you delete it:
Before backing up your 10.7...
...Users folders of: Music, Pictures, Movies, Documents etc., (not Library) off the 10.7 machine to a storage drive (not TimeMachine):
Export any bookmarks, passwords, product keys or other vital information that may be stored elsewhere on the computer.
Write down the names of the users accounts, case sensitive, this will be vital to restoring files and not have permissions issues.
Eventually newer versions of iTunes and iPhoto are on newer OS X versions, you need to take some special precautions so your newer version altered files will work in the older versions of these softwares in 10.6
In the iPhoto Library, right click on it and "show package contents" inside is a folder called Originals, duplicate it and copy it out.
In the 10.7 iTunes, go though all your playlists and assign a grouping (or or tag) to the song files themselves that represent the playlist they belong too. This will assist in rebuilding your playlists in case the 10.7 iTunes database file won't work in the 10.6 version.
Any other program that has your data trapped in a only later OS X version format you need to extract into a more flexible/neutral format so it can be used in earlier versions of that software.
Hit your Bootcamp and backup any data in there as well. You can't keep Bootcamp, it will have to be rebuilt in 10.6
Backup your 10.7 user(s) data off the machine to a storage drive
We don't want to copy the hidden 10.7 "User"/Library folder containing those users settings to the 10.6 machine later, so don't copy the entire 10.7 User folder(s).
Copy just the 10.7 Users data folders of: Music, Pictures, Movies, Documents etc. that are visible (and have data in them) into new named User folders on the storage drive.
TimeMachine connected to a 10.7 drive won't work to restore files to 10.6, don't rely upon TM for this or you'll be sorry (and back on 10.7 again)
If you need to learn about storage drives here first, also if you decide you want a clone of the 10.7 OS X partition for safekeeping, extra backup, decision change, now is the time.
So at this point you should just have your 10.7 User files folders (Music, Pictures, Movies, Documents etc. no Library) on external storage drive, now disconnect ALL drives, make sure they are labeled so you know what is what. We don't want any mistakes to occur and you erase the wrong drive.
The same methods outline below can be used on a blank external drive with a GUID and OS X Extended journaled format to install 10.6 there if you should desire.
The same methods outline below can be used on a 10.7 internal partition with a OS X Extended journaled format on the second partition to install 10.6 there if you should desire, but just the second partition is erased (if need be) not the entire drive.
Erasing the boot drive:
Before erasing the internal boot drive, this is the last chance to copy the Lion Recovery HD to a USB thumb drive before it's gone, skip to the bottom of this User Tip to do that first. However it's not necessary to get back Lion as you can reinstall Lion + Recovery HD again from the app store like before when you upgraded 10.7 over 10.6. But having it on USB will allow one to install Lion on a external drive, as the Recovery HD is used to download Lion from Apple's servers.
With all other drives disconnected your ready to erase Lion completely off your boot drive, and you do need to erase the ENTIRE drive.
Hold c (or option key) boot off the 10.6 installer disk (or hold option key boot off the 10.6 clone drive) and under the Utilities menu is Disk Utility.
Select the entire internal boot drive on the far left, it will have the drive makers name and size.
Do not select the indented names, those are just partitions on the drive, you need to erase the entire drive to catch hidden 10.7 partitions and rebuild the partition map back to 10.6 version.
Click Erase > Security option > Zero All Data (will map off any failing sectors) go watch a movie, it takes a bit and improves your hard drives reliability and read speeds. (SSD no need zero, just erase).
Check under Partition: Options that you have a GUID and Format: OS X Extended Journaled, if not change it to those and apply.
Quit Disk Utility. You will be back into the 10.6 installer.
Restoring Snow Leopard 10.6
If your using your option key bootable 10.6 clone, use the cloning software to reverse clone 10.6 onto the internal drive then return your files from the storage drive. Your done, see you later.
If you have a 10.6 TimeMachine drive (that was never connected to a 10.7 machine), then install 10.6 with the boot 10.6 disk and upon reboot connect the 10.6 TM drive, use Setup Assistant to restore from TM, then return recent files from the storage drive. Your done, see you later.
If your Mac came originally with 10.7, then the firmware will not let you boot from the 10.6.8 combo disk you made or any other pre-10.7 boot disk, you need another another Mac that can boot the disk and Firewire Target Disk Mode to get past the firmware and install it on the target Mac. I can't help you any further here, your on your own, consult the thread where you made the 10.6.8 disk.
Ok, I got rid of the rest, now for you who just erased the internal drive via the 10.6 disk and need to install fresh.
Quit Disk Utility and install 10.6. from the installer disk.
Reboot, setup and use the same user name as on the 10.7 boot drive. Wrote that down I hope, right?
Use Software Update to get up to 10.6.8 fully. This is very important to do or you'll have problems later.
Install all programs from fresh sources and compatible with 10.6.8
(if Gray screen boot hang, hold shift key upon boot and then update software or uninstall)
If you had 10.5 previously on the machine, you'll notice your iLife is missing, this is because the 10.6.3 disk doesn't have it, only the machine specific disks do that come with the Mac. However you can either buy the 10.6 iLife from Apple or use Pacifist from CharlesSoft to extract the 10.5 versions from the 10.5 install disks. You paid for it so it's yours.
Once you have all your third party software installed (as best as you can of course) next is to hook up your 10.7 storage drive.
Return the contents of those user files folder to their respective folders in each account
Use the 10.6 version of iPhoto to import the 10.7 Originals folder extracted from the 10.7 iPhoto Library previously.
Also with 10.6 iTunes, import the music from the 10.7 iTunes folder and later remake your playlists based upon the grouping tags you applied while in 10.7
Tweak things so your comfy again on the snow kitty.
Hard drive performance tip
If you installed nearly exaclty in the order as I have outlined above (OS X, update, programs, user files), and keep your hard drive below 50% filled, it will operate at peak performance forever until it dies, or you damage it by moving it around while it's operating.
Your hard drive performance will suffer greatly the more OS X boot partition data is written on the second 50% of the drive, even if you remove some to get under 50%.
If there is any data on the second 50% of the drive, the heads have to travel there to get it, use the smaller sectors and that slows read/write performance on hard drives (SSD no need to worry).
Whatever you do, don't go past 75% filled on boot drives on Mac's as your pressing your luck.
Start using a storage drive to offload data your not using.
Congratulations, your back on Snow Leopard!
Now to avoid this chore of a complete rebuild ever again, make a couple of 10.6 bootable clones!
Even clone Windows with Winclone! Read more here.
Restoring Lion again
Already? Simply log into AppStore and hold the option key and click on Purchases to redownload the Lion installer, it will rebuild the Lion Recovery HD like it did before.
If you want the Recovery HD on a bootable USB thumb drive to restore with, you can learn about that here