How would you like to gorge yourself on free iPhone ringtones like a hungry pig in a cake shop?  Well, with a little effort you can take advantage of Apple’s generous 30-second preview functionality in iTunes to get yourself all the freshest music (or, if you’d rather, all the classics) without paying a dime.

It basically involves importing a playlist of track samples into the iPhone as custom ringtones, using the existing guide, which the iPhone will then happily play seeing as it’s linked to your iTunes account.

Here are the instructions, care of TUAW:

1. Create a new playlist. Drag unpurchased songs from the iTunes store into your playlist. The songs will retain their “Add Song” buttons and their price within the playlist.

2. Export your playlist. Select the playlist in the sources column. Control-click/Right-click the playlist name and choose Export Song List from the pop-up menu.

3. Save the playlist as plain text. Select Plain Text from the Format pop-up and save the playlist file to your desktop.

4. Open the playlist file. It is a tab-delimited file of columns, so you can open it up in Excel (my preference, make sure to option-drag the text file onto the Excel icon) or a text editor like TextEdit.

5. Locate the file URLs. Each file URL appears in the final Location column for each line. Copy the URL.

6. Download the files. In Safari 3.0, open the Downloads window (Windows->Downloads). Paste the URL into the Download window and allow the file to transfer. Your computer must be authorized to your iTunes account. You may want to try playing back the file in QuickTime Player just to be sure it downloaded correctly. If you’re not a Safari 3.0 user, use your favorite alternate such as curl, wget, or so forth.

7. Rename. Give the file a more meaningful name than, for example, “mzi.rwgtaash.aac.p.m4p”. Retain the .m4p extension.

8. Upload to the iPhone. Use your favorite method (iphoneinterface, sshfs, sftp, whatever) to copy the file to /Library/Ringtones on your iPhone.

9. Select the ringtone. On the iPhone, navigate to Settings -> Sound > Ringtone and select the new file. The ringtone will play back as you select it. Please note that some newer releases (including Nicole Scherzinger’s Whatever U Like–thanks Drunk Dwarf) do not work as ringtones. I’m not sure why.

Congratulations, not only have you added a new 30-second custom ringtone to your iPhone, but iTunes usually picks the best 30 seconds of any song for its preview. Enjoy.

GeekNote: If you’ve got curl installed on your iPhone, you can curl the URLs directly to /Library/Ringtones.

As ever, if you try it let us know in the comments as to how you get on!