I'm the iTunes music pharmacist for my family's iPod and iPhone music libraries. In short, what this means is that, whether it's due to "getting sick of hearing the same thing over and over when I'm running" or wanting a new song or album on their iPod, they come to me when they want their music refilled. Which is fine and dandy. I'm more than happy to do this, and have my trusty ol' 17 inch iMac loaded with everyones' music on it (except mine, which is loaded on my MacBook Pro).
From the 3rd generation iPod classic to the iPhone 3G S, and from The Arcade Fire to ZZ Top, there exists an eclectic iPod and music mix in my family. How I would sync music for each person varied on a case-by-case basis, and usually took shape in syncing specific folders, playlists, smart playlists, and genres. For the iPod shuffle, however, I would typically click on the "music" within the device and choose playlist-based "autofill."
After spending a half hour or so creating a size-limited smart playlist with random songs, I said to myself, "Self, there's gotta be an explanation for this. Surely, the folks at Apple wouldn't leave out something as important as autofill."
And they didn't. They just put it in a different place for the third generation shuffle, and have now extended the feature for all iPods and iPhones.
To enable autofill on your iPod or iPhone, click on "summary" tab for your device (in the "device" section of iTunes). Then place a checkmark on "manually manage music [and videos]."
Now here's where I got lost and confused, and I don't want you to pull out your hair like I almost did mine just because I didn't read the Apple support forums carefully enough. Except for first and second generation iPod shuffle models, the autofill option for all iPods and iPhones is activated by expanding the device (by clicking on that little triangle to the right of it) to reveal its media folder hierarchy. Then, you'll want to click on "music," and now you'll see the autofill option in the bottom left hand corner. The whole time, I thought it'd be located on the "music" tab within the device, much like how it's on the "contents" tab on the second generation iPod shuffle.
Autofill is now available for all iPods and requires iTunes 8.1.
The more you know...Dun, dah, duh, ding.