The AppStore does not allow you to easily save multiple versions of an application.  Once a developer updates an application in the AppStore, older versions can no longer be downloaded.  There may still an opportunity to backup an old version if it has not yet been updated from from your iTunes library.  You also may be able to retrieve the IPA from your device by syncing it back to iTunes (if your iTunes library does not contain a newer version, thus updating the device rather than transferring a purchase back to iTunes).

No guarantees.  This process is unsupported by Apple and by All About Jake, LLC: Your mileage may vary.  There is no guarantee that this process will work, and if you do it wrong, you might wind up accidentally updating to the newer version anyway.  However, if you are careful about how and where you update your applications from the AppStore, and how you sync to iTunes, then you should be able to save a backup copy.

Also, with Apple’s introduction of iCloud, automatic updating, and wireless syncing, this all becomes that much more difficult in the future.  I recommend backing up applications shortly after they are released, rather than when there is already a version waiting in the AppStore for you to upgrade.  This way, you know that an automatic process, errant sync, or accidental upgrade won’t leave you without a backup of an old version!

Information

This must be done before you update your application from the AppStore to the new version, or the old version will be replaced.

Please review the following sites for information on how to backup and restore old versions.  Make sure you fully understand how this work before you proceed:

• http://reviews.cnet.com/8301-19512_7-10345683-233.html
• http://www.tuaw.com/2011/01/28/ipad-101-reverting-to-an-older-version-of-an-application/

In Summary:

1. Locate your application in iTunes under “Library -> Apps”.  If it is not there, you may be able to sync your iOS device to transfer the application back to your iTunes library, but be sure that your iTunes library does not contain the updated version or you may wind up updating the app on your device.
2. Right click and select “Get Info” to see the information about the application in your iTunes library, and see the version number (for reference)
   
3. Exit the info dialog.  Then right-click the app and select “Reveal in Finder” or “Reveal in Windows Explorer”  This will show you the file on your hard drive.
   
4. Copy the file to a safe location.  Congratulations, you now have a backup of the application file in a safe location.  When iTunes updates to the latest version of the application, you’ll have a copy of the old version safely tucked away.