Read this:
Defensive Techniques by Position
After reading this I now know the importance of shifting my DBs, something I never did before except to shift up for bump and run.
The info is for reallife but you know it will work in this game. It tells you that for CBs you want to funnel the WR outside in man to man coverage and inside for 1, 2 and 3 deep coverage.
So, if I call a play with man coverage underneath then I shift them inside the WRs. If I call zone for my CBs I shift them outside the WR.
I was playing with the vikings subpar secondary and picked plummer off twice in the first half doing this alone.