Very hard to do with just regular mgt. You need lots of LMB to check the crappie down which makes it hard to raise large LMB.

Here is a out of the box thought. Electroshock and take out every crappie you can and leave all LMB. After next spring's spawn (both crappie and LMB) if crappie numbers look to be under control then electroshock again and take out LMB to balance #s and sizes.