Repartition my existing hard disk
Make sure to back up all of your data first, just in case.

Next, create a USB Recovery HD (at least 1GB) with these Apple forum instructions:


  Create a USB thumb drive Recovery HD using the Lion Recovery Disk
  Assistant. Download this and install it. Then, insert a thumb drive
  and create your bootable Recovery HD. Last, boot from the USB thumb
  drive and from inside the Recovery HD on that, try the resize.


You should be able to resize your main partition after booting from the USB Recovery HD.

Another option is to use the Recovery HD on your Mac, as noted in the solution that @bmike suggested in another question - boot to your Recovery HD and use Disk Utility there to resize.

The solution that you've mentioned will work, but is a bit more work.