The video below shows the hands-on basics of sealing your ducts. I like the part where this HVAC expert uses toilet paper to find leaks in the system. Then, he shows you how to seal them up. This is not just theory or book learning, folks. This is the real thing.
So will you spend a few moments of your time on this? The investment could be great. Getting your ducts sealed sure beats losing your money to needlessly high heating and air conditioning costs. And you know that condensation that forms when air leaks out of the system? That's not a good thing.
Video from Train2Rebuild.