A good guide is available in a recent Lifehacker article. It's for triple-booting, but you can scale it down to dual-boot Linux and OS X. Excerpt:
We won't be using Boot Camp to partition our disk, mostly because we don't need to. Since we're triple-booting, it's easier to see it all at once, rather than let some tool do it for us (if you've already installed Windows using Boot Camp, though, that's fine—just ignore the Windows parts of this step). Open up Disk Utility, click on your main drive (the very top option in the sidebar) and head to the "Partition" tab.
Go through the rest of the [Ubuntu] installation until the last window, where it's ready to install and asks you to double-check everything. Hit the advanced button. Make sure "install boot loader" is checked, and change the device for boot loader installation to the same partition on which you installed Linux (in the above case, /dev/sda4). Hit next and let 'er rip. When it's done it should restart your computer and your rEFIt boot menu should contain an option for all three operating systems. If you can successfully boot into both partitions, congratulations! You've got a working triple-boot system.