Try this, I find these kill your traps. When your doing Shrugs (BB or DB): With the weight completely at the bottom: 1. Lift weight half-way up, pause for a second. 2. Lift it as high as you can, pause for a second. 3. Lower it back to the half-way point, pause for a second. 4. Lower it completely, pause for a second.
If you can normally do 12 reps with xxlbs, your probably only gonna be able to do 8, maybe 10. So go a bit lighter.