^^ what Stella said! ^^
In the evenings, when we were watching television, whenever the ad break came on, i would leave the room for the whole 2/3 mins and shut the door. Potter about, make a cuppa etc, then go back in the room, all the time ignoring the crying.
She eventually got used to being left alone, as she knew i would always come back. Gradually i then increased the time she was left alone, she now has no issues.