The Montgomery County Commissioners have taken steps to preserve more undeveloped land.   They approved a $342,000 open space grant at their last business meeting that will help the Montgomery County Lands Trust preserve the 78-acre Rogers-Heister property in Upper Salford and Marlborough Townships.   The property is located off Route 63 near Sumneytown and features an 18th century mansion that the former owners have maintained over the years without significantly changing its look and feel.