For large software downloads you might consider grabbing them on someone else's dime--particularly if you work for the kind of generous company that has bandwidth to spare and doesn't mind if you grab a couple of gigabytes of data and shove it on to a USB stick during your lunch break. Coffee houses aren't so good for this kind of thing as their connections are generally pretty slow (and only get slower when many people are sharing a connection to do this kind of thing).
Finally, consider keeping a copy of the Mountain Lion installer. By default, when you download and install Mountain Lion, the original installer file is deleted. This means that if you want to reinstall Mountain Lion or install it on another Mac you own, you have to download it again, which hacks around 4GB out of your monthly data allotment. Download it the one time and move it out of the Applications folder. When you do this and install OS X, the installer will remain on your hard drive.
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