If you were one of the many to leap into the Mac App Store and download Mountain Lion, I hope you were ready. Once you’ve upgraded to Mountain Lion, there’s no way to revert back to Lion through the Mac App Store.
Do yourself a huge favor and make sure all of your favorite apps are compatible with Mountain Lion before upgrading. The only way you can downgrade to Lion would be outside of the Mac App Store using a backup copy or a cloned disk image.
If you haven’t upgraded yet and are unsure whether or not all of your apps will be compatible with Mountain Lion, you do have a fallback option. There’s a great free program called SuperDuper! which will allow you to create a bootable clone of your hard drive to a disk image or an external drive. If one or more apps on your Mac aren’t compatible with Mountain Lion, you can use SuperDuper! to restore the bootable backup. Check out SuperDuper! to find out more details on the software and how to use it.
I ran into a similar compatibility problem when Lion was released. The situation was actually disastrous, and I was lucky to have a physical Snow Leopard disk. Apple did away with physical disks when Lion was released, making things a bit more complex in the restore department.
When I upgraded to Lion last year, I found that a few key apps I used on a daily basis would no longer launch. I strongly suggest checking with the developers of any important apps on your Mac to make sure everything is Mountain Lion compatible before upgrading.
Source: Cult of Mac