1. Turn on USB debugging under application settings menu.
2. Plug in USB cable to your phone and the Mac.
3. Pull down notification window and mount the device.
4. Download the rooting file from the following link and rename it 'update.zip'.
http://theandroidsite.com/wp-content/uploads/2010/07/SGH-959-Root-Update.zip
5. Drag the 'update.zip' file onto your phone mounted on your Mac.
6. Unmount SD drive from the Mac and then unplug USB cable.
7. Turn your phone off.
8. Hold volume up, volume down keys and push the power button and hold all three buttons.
9. When you see the white screen with AT&T logo appear, immediately release the power button only but keep holding the volume up and down buttons.
10. From the service menu, choose 'reinstall packages'.
11. You phone should reboot. Now your phone is rooted!
12. Go to Android Market and download 'RyanZA's One Click Lag Fix' app and run it.
13. Click 'Install EXT2 Tools'
14. Click 'OneClickLagFix V1+'
15. You are done.
RyanZA's one click lag fix app might actually unroot your phone also. I would try that first.
To unroot, download 'terminal emulator' from the market and type the following command.
su (enter)
rm -r /system/app/Superuser.apk (enter)
rm -r /system/xbin/busybox (enter)
rm -r /system/bin/su (enter)
reboot (enter)