It seems as though Apple is designing a lot of new Mac and iOS features from the perspective of people whose internet access is the equivalent of an all-you-can-eat buffet. Photo Stream is a perfect example. The feature, built into Apple's new iCloud service, automatically uploads photos taken on any of your devices to iCloud, which gives you near-instant access to them on all of your devices.
It's a great feature, and it's already streamlined the process of getting photos from my iPhone onto my iPad and Mac. But it was only this morning that I realized with grim horror what might happen if I imported 8 GB of RAW files from my DSLR into Aperture. With Photo Stream's default settings, those multiple gigabytes of data would be uploaded to iCloud automatically -- and since I live in an area with strictly metered data and very expensive internet access, a single import session in Aperture could cost me dearly.
Fortunately, both Aperture and iPhoto allow you to disable automatic uploading very easily. Go into the preferences for whichever application you're using and select the Photo Stream pane. Once there, uncheck the box next to "Automatic Upload." As easily as that, your photo manager of choice will no longer send hundreds of photos into the cloud the next time you connect your digital camera.
The followup question you must be asking now is, "That's great, but what if I want some of my imported photos to go to Photo Stream?" Fortunately, Apple made it easy to manually upload photos to Photo Stream. Just select the photos you want, then drag them to the Photo Stream item in the sidebar. You should see a green plus sign when you hover over Photo Stream. Release the photos, and they'll be uploaded into the cloud.
Whether you're trying to avoid massive data charges or simply don't want hundreds of photos clogging up your Photo Stream all at once, it's quite fortunate that Apple has made it so simple to disable automatic uploading and manage your Photo Stream uploads manually instead.