… The Open/Closed principle is probably the most misunderstood of the 5 SOLID principles. However it’s the one which when properly applied can save you more development effort through good architecture than any of the others.
Alan has written a series of some good class design and implementation guidelines post. URL: Object Design and Implementation
Rule #1: Practice Drawing Make Good Classes
Rule #2: Documation!
Rule #3: Alan’s Class Template (“ACT”)
Rule #4: Never Walk, But Fly!!!
Rule #5: Always evaluate How We Fly!
Rule #6: No Private Variable Shall Have Direct Access to the Class Public Interface
Rule #7 & #8: All about Numbers!!