In iTunes 8.1, which you can download here, Apple has made it possible to use the Autofill feature, previously only available for iPod shuffles, with all iPods, even the very first model of the iPod, the Classic, as well as the iPod Touch and yes, even the iPhone.
Autofill lets you use iTunes 8.1 to automatically "fill" your iPod with music. It's quick, and fun, because you never know exactly what music you'll be hearing. If you have a large collection of music, Autofill is a great way to keep fresh music on your iPod without spending hours and hours selecting your music track by track. You might want to create a special playlist first that iTunes will choose songs from when you use Autofill. Otherwise, iTunes will draw on your entire iTunes music library. It's pretty easy to use the new Autofill feature, though it's not what I'd call intuitive. For one thing, you can't even see the Autofill option unless you set iTunes to manage your iPod manually.
The instructions that follow are about using iTunes Autofill with non-shuffle iPods.
Warning: Autofill will overwrite non-iTunes data, including Custom Ringtones on iPhones.
Connect your iPod to your Mac as usual. First, we'll make sure that you have iTunes set to update your iPod manually. Once iTunes 8 launches:


Now, look on the far left of the window, at the sidebar known as the iTunes Source list.

Look at the bottom of the iTunes window, on the far right. You should see a new pane at the bottom, with two new buttons; Settings and Autofill.

You need to specify the setting you want Autofill to use. Click the Settings button on the bottom right of the iTunes window. You will see the Settings window:

You have these options:
Replace all items when Autofilling—This will overwrite all the current music. Deselect this if you want Autofill to fill only the remaining space on your iPod.
Choose items randomly—If this is selected, then songs will be added at random; otherwise, songs will be added in the order in which they are listed.
Choose higher rated items more often—If this is selected, songs you you have rated more highly will be more likely to be included in Autofill song selections.
Reserve space for disk use—This slider allows you to choose how much space is reserved for file storage.
The pop-up menu allows you to select a Playlist, rather than your entire Music library, for Autofill to use when adding songs to your iPod.
Click OK when you have made your selections. These will be remembered and used in future Autofill sessions, until you change them.
Now, click the Autofill button at the bottom right of the iTunes window, and iTunes will autofill your iPod.

You'll see a dialog warning you that iTunes is about to replace your music with new music. Think carefully before clicking OK; there is no Undo option for this.