Trim mondo grass in late winter by mowing it. Cutting off the tops removes any old leaves that might have been damaged by winter weather; it will grow back fuller than before.
Dig up portions of the grass that develop outside of the designated area. Use a spade to dig around the clumps to ensure you get some root with the removed portion.
Slice into a clump of grass to divide a clump that has gotten too large. Cut into it with your spade and dig out half of it. You can replant the portion you removed in another location.