I know this is going to sound weird, but when you are broadcasting you have to make sure that all of your apps that use sound are pointed to the same output or input. I had the same problem early on and discovered that skype preferences had a different setting then my boinxtv or cam twist. I use soundflowerbed to configure soundflower options and output sound so I don't get an echo, but I have to make sure that all of the settings are the same. I have had to keep a checklist to remind myself of the settings before I go live, during interviews or live on the Internet.
I use Audacity to edit audio and even those settings have to match. I was bouncing between Audacity and a desktop video file (I was capturing the audio from the video) and monitoring it by changing the system preferences between speakers and sound channel 2. These settings can make you crazy, but once you get the hang of it you should be able to capture audio, play it through your system to your livestream channel and monitor through your headphones without an echo. Remember though, I use BoinxTV, Cam Twist and Procaster. Soundflowerbed will help manage sound and it's free.