Heavy deads from just below the knee really did it for me. Shorten up the range of motion so it's more of a back/posterior chain exercise.
Also DB shrugs, but try going about 3/4 as heavy as you usually would and really get the full ROM pulling up, then back, then up some more.
Less conventional, but a good way to get some blood in there, are trap flies. Use fairly light dbs, lean forward a bit at the waist (standing), roll the shoulders forward, then do a lateral-ish movement but instead of shooting straight out to the sides, try 45 degrees in front. It's hard to explain, but there's a good Youtube tutorial I'll try and find later.