While I keep my 5th generation iPod filled to capacity and use it solely as a music player in my car, I can’t stuff 20 gigs of audio files on my iPhone 3G. And having the same set of selected songs on my iPhone can get a little stale, especially when I don’t sync the device for several months.

So in order to rotate different music on my iPhone, I’ve started using the Autofill feature of iTunes, which is somewhat hidden in the iPhone and iPod Touch setup. It takes a few clicks to find the settings. But here’s how to get it done. (Note: this process is different for 1st or 2nd generation iPod shuffle users. Autofill for those devices are clearly evident.)

The purpose and benefit of Autofill is to have iTunes automatically add and/or replace songs on your portable Apple music playing device rather than you having to manually select playlists from your iTunes library. It also gives you the ability to determine how much memory you want devoted to songs on your device. This is crucial if your iTunes library is very large and all your songs can’t fit on your device. So you may decide that each time you sync your device with iTunes that you want to Autofill it with say 3-4 gigs of music.

First off, in order to use Autofill, you need to manually manage your music on your iPod (when I say iPod, I”˜m also referring to the iPhone and iPod Touch.) So when you connect your iPod to iTunes “˜09 for syncing, you’ll want to select the device in iTunes and click the “Manually manage music and videos” box under Options.

Next, you want to select Music under Devices, which will show all your music in iTunes. From there, you have a few important decisions to make. First off, you need to decide if you want Autofill to choose music from your entire iTunes Music library or if you want it to select from a particular playlist of songs. Click on “Autofill From” at the bottom of the main iTunes window and make your selection.

Now click on the Settings button on the right side. This is where it gets somewhat tricky. Where it says, “Reserve space for disk use,” is a little confusing. You want to first decide how much memory space you want left (reserved) on your iPod after Autofill adds music to your device.

In other words, say you want about 311 megabytes of free space for new applications, videos, podcasts, etc, then you will want to set the “Reserve space disk use“ slider to that amount. In my test, the numbers didn’t always match up, but they were fairly close. When I set the number to 311.3 MB, it filled my iPhone with 4.56 GB of songs, and left about 365.1 MB of free space for other stuff. That’s okay, because I don’t add as many apps and video files as I used to. I just want to make sure I have a good variety of songs from my all jazz playlist. If video files and podcasts are your primary media use for your device, then you might allot a gig or more of free space.

Lastly, you need to decide if Autofill will do the following:

“¢ Replace all items when Autofilling (It will mainly replace all songs, not other stuff.)

“¢ Choose items randomly.

“¢ Choose higher rated items more often.

I chose all three.  Simple. If you don’t select “Choose items randomly“, you will not be able to select “Choose higher rated items more often.”

After making selections, click OK and go back to the start page of your device and click Sync. Autofill will do its thing.

If the changes you make don’t come out as you wish, don’t worry; you can always revert back to the previous setup of selected playlists.

So how do you prefer to update your iPod? Manually with selected playlists or automatically Autofilled?