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Since you're here, maybe you already installed the beta OS X Mountain Lion 10.8 Beta/Developer Preview and get some troubles with it. You notice that some things that you really need won’t work any more (especially some 3rd party programs and utilities) due to incompatibility issue. Or your machines turn to be a little slower than before after upgrading. Or maybe you haven't build the habits of iOS to desktop feature. Finally you may consider downgrading Mountain Lion to Lion maybe a good solution. If so, you have below 2 options.

Method 1: Downgrade with Time Machine back up

If you have good habit of backing up important things before hand, you may have a stable back up from time machine which still has a 10.7 Lion system in it. Then we begin from here.

1. Reboot your system, press the option” key while the system starts up and select Recovery” or anything that has recovery in it.
2. Select Restore From Time Machine Backup” option from the menu.
3. Go through the wizard and you’re done.

Note: You can also boot from an external HDD clone, format the internal HDD and clone Lion back. If you don’t have machine time or external HDD back up before the upgrade, then you can try the other way described below.

Method 2: Downgrading by erasing disk and Internet recovery

1. Reboot your system.
2. Press option” until you can choose your boot drive.
3. Select the recovery partition even it says only for OS Mountain Lion 10.8.
4. Now erase your whole disk including the current kernel (kernel means the partition which contains your OS). You’ll have to choose your disk utility in the recovery mode.

   

5. After deleting all the data, select Reinstall Mac OS X. (Make sure you’re connected with an Ethernet cable or WIFI which uses the WPA/WPA2 authentication. The Wi-Fi menu item is in the upper-right corner of the screen. Click the icon to display all available Wi-Fi networks. Click your preferred network name and, if needed, enter a username and/or password. )
6. Your system will automatically check which system it needs and reinstall it (usually the pre-installed version) from the servers of Apple. For example, if you bought your Mac with Lion installed, then Lion will be reinstalled smoothly; if you bought Lion from the Mac App Store, you will be prompted to enter the Apple ID and password you used to purchase Lion.)

Important: 1) Make sure that all your partitions are erased before reinstalling; otherwise it may doesn't work or will reinstall OS X Mountain Lion instead. 2) Not all Macs running OS X can benefit from Internet Recovery, but newly-released Mac starting with the Mac mini and MacBook Air can. If your machine doesn't suite with above occasion, you can choose to adopt Apple Recovery Disk Assistant, which helps to create a bootable OS X Recovery drive using your own USB drive that does everything to recover your Mac, including reinstalling OS X from Apple servers.