This is what works for me. After I installed the new iWork updates I created a folder on an external drive & named it "other applications." I then moved the new iWork apps to this folder & renamed the applications Keynote 6, Pages 5 & Numbers 3. I have icons for two versions of Pages & Numbers (I rarely use Keynote) in my Dock. Even with two versions of Pages & Numbers running, double-clicking an existing file opens in Pages 4.3 or Numbers 2.3. This should also keep the Mac App Store from nagging you to update.
Here is the solution posted by Peter in iWork Tips 'n' Tricks:
Making Pages '09 the default whilst keeping Pages 5
1. Move any new iWork apps in another clearly labelled folder
2. Menu > Apple > System Preferences > Spotlight > Privacy > drag the above folder into the list
2) Move the new iWork folder into the trash (temporarily)
3) Select any Pages file in Finder > Menu > Info (cmd+i) > Open with: > Pages '09 > Change all
4) Re the new iWork folder from the trash (right-click > Put back)
Pages 5should no longer appear as a default App, and you won't have to keep a copy in a separate drive or dmg.
IMPORTANT: Make a note of where and how Pages was installed for the next App Store update and temporarily move it back there before putting it away again in the folder concealed from Spotlight.