What can I do to help my German Shepherd who has separation anxiety...?

My GSD is just over a year old and is awesome. She has been to obedience cl but the only problem she has is separation anxiety. An couple of examples would be I put her in a sit stay and when I walk away she barks like I'm not coming back and second, if she can see me and not get to me (ie. screen door or on the other side of a fence) she starts to bark and will jump a 6ft fence. She doesn't destroy things in the house when my husband and I go to work and she's fine alone, its just when she can see me and not get to me is when it becomes a problem. What can I do?