Here are some videos my students have found helpful and I never tire of watching. A study on the form through the years with Master Tung Kai Yang doing the the first section of the slow set, the second section of the slow set, and doing sword.
Retreat and Seal
This is one of the best lessons demonstrating retreat and seal by Grand Master Tung Kai Yang. Simple, yes. But not easy! Have fun.
Master Tung Kai Yang teaches in Los Angeles. website: http://www.tungkaiying.com/master.shtml
Other Taiji Quan schools in the Tung lineage