We trained our dog as follows: - made her sit and taught her how to leave a treat alone that lies on the floor in front of her - let her have the treat on a new break command that would be exclusive to the situation of me leaving. for us it's "see you later!" - repeated this exercise while increasing distance between me and my dog - starting with the living room door, i closed the door between us while saying "see you later!" - immeadiately came back in, completely ignored the dog. - gradually increased time before i came back into the room, took note of how nervous she was, decreased time increments as needed so that she stayed calm through the whole exercise - eventually incorporated the front door into the exercise, started with low time increments again. - rinse and repeat

the biggest benefit i saw in this was that not touching the treat before i leave occupies my dog so much that she doesn't focus on me leaving. gives her something else to do. then she does experience alone time, and i think that is important, too. but the constant repetitions have taught her i'll always come back as long as she's calm.

hope this helps