Not being able to download willy-nilly as before, I had to come up with some strategies for optimizing my download quota:
Download off peak: If you have an unlimited period as I do, download as much as you can during that period.
Watch out for updates: Don't download updates to OS X or iOS automatically. On OS X, go to the App Store pane of System Preferences and uncheck Download newly available updates in the background. You'll still want to be notified of updates, but you certainly don't want to download what may be several gigabytes of updates without warning.
When possible, download updates from the Apple website, rather than the App Store app. You can only do this for certain updates, such as iTunes, or general OS X updates, but if you do, you can transfer the update disk images to any other Macs you have, so each computer doesn't have to download the same content. When you update apps from the App Store app, you have no choice: You can't copy those apps to another Mac. So each Mac will have to download its own app updates.
Don't double-download: Don't automatically download purchased apps and media to all of your devices. There are settings in the App Store preference pane, and in iTunes (iTunes > Preferences > Store) for automatically downloading apps and music to your various Macs. You should turn these off, so you don't end up downloading that 1GB game on your laptop, after you've got it on your desktop Mac. Download all iTunes content to one Mac, then copy it to other Macs, if you have more than one, and sync to your iOS devices.
Take advantage of other Wi-Fi networks: If you update iOS apps on any of your devices when on a Wi-Fi network away from home — at work, say, or at school — copy those updates to your iTunes library. To do this, connect the device to your Mac, right-click on it in iTunes, then choose Transfer Purchases. If not, you'll end up re-downloading the same apps at home. And if you have a laptop, try and grab your updates on someone else's dime.
Say goodbye to streaming: Do you like Netflix? Do you use Spotify or iTunes Radio to listen to music? You will have to think twice about using any of those streaming services when you're on a limited bandwidth diet. Depending on your data allowance, these services could cost you a lot. Check how much data each one uses, and decide whether it's worth keeping. Music uses less bandwidth than video, of course, so you may be able to keep your favorite music service with little impact to your quota. Just don't leave it running all the time if you're not listening.
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