Lower the tempo, shorter passing, retain possession & try a more defensive mentality like defensive or contain.
Side line talks such as concentrate and demand more can help too.
Quite clearly, this isn't complacency. It is tactical. The opposition either decides to attack more after half time or they park the bus. It seems your tactic isn't the best against those teams.What should I do to stop my players from being complacent when leading by 2 or more? Almost every game that I have the domain at 1st half, my players stop the great performance at 2nd!!