Keeping 10.x Updates
Update April 2015: I was able to retain a copy of the Yosemite 10.10.3 Beta update via this same method. The trick is the updates do get erased if you let App Store install and reboot. You generally have 60 seconds after the download completes and before Finder shuts down to locate the update file and copy or duplicate it.
Back before all Apple updates came to be distributed through the App Store, it was easy to retain software updates. You just told Software Update app to Keep Updates through its menu system.
There are distinct advantages to retaining updates instead of redownloading them every time. Bandwidth, which Apple seems to think we all have in abundance, is a large one, especially considering many updates are measured in gigabytes these days. But perhaps you also want to keep an update for use in AutoDMG, InstaDMG, Munki, or Casper Suite.
With the advent of App Store, these updates are notably harder to retain. But if you’re quick, you can still keep them. Follow these steps:
1) Initiate the download of the update using App Store.
2) Once the download is complete, look in /Library/Updates/[random #]
OK, the number probably isn’t random, but you’re unlikely to find out Apple’s numbering system, so just look at the most recently updated numerical folder inside /Library/Updates. Your .pkg will be there waiting for you.