Notes I have carried on my PDA or in my Franklin planner for years: Info Mac OS X CD Sessions

Info Mac OS X CD Sessions
Many people have asked us how to "reuse" a regular blank CDR, so they  
can burn data to it multiple times. This is typically called session  
burning. You can purchase dedicated applications do this (such as  
Toast or Disco), but you can also use Disk Utility which is built  
into OS 10.4. It takes a little planning to make this work, but it  
can help you cut down on the number of CDs you have to burn (and buy).

This great tip comes from Apple:

First create a new folder and give it a descriptive name. Now put the  
files you want to burn into that folder. Go to your Applications  
folder and open the Utilities folder. Double-click on the Disk  
Utility application.

After it launches, in the menu bar at the top of the screen choose  
File > New > Disk Image from Folder. When the "Open" dialog box  
appears, find the folder you created earlier with the data you want  
to burn, and click "Image." A "Save" dialog appears. You can leave  
the name as is or choose a new name (ignore the other options in this  
dialog box). Click Save. In a few moments, a disk image of your  
folder’s contents will appear in the list on the left side of the  
Disk Utility dialog. Click on that icon, and then click the burn  
button at the top left of the Disk Utility application.

When you click the Burn button, a dialog will appear asking to insert  
a disc. Do so, then click once the blue downward-facing triangle on  
the right side of this dialog to show more options. Click on the  
checkbox for "Leave disc appendable." Now Click the "Burn" button.

Your data will now be written to that CD. To add more files later,  
just insert that same CD and use this same process all over again.  
Note that when you get to that final burn dialog, the button won’t  
say “Burn” this time, instead it will say “Append” because you’re  
adding these files to the same disc.

Don’t forget to remove the files you already burned to this disc from  
the folder you created above (and the DMG file it creates) before you  
make your next disc image.

Note that this is different than using a CDRW (compact disk re-
writable). A CDRW can be erased and re-burned multiple times (however  
it's not recommended to do this more than 10 times per disk).  
However, CDRWs don't work in all drives, and are expensive. This is  
tip is designed for cheaper and more durable CDR's.

http://www.discoapp.com/