SOLID
Single responsibility principle
A class should have only a single responsibility (i.e. changes to only one part of the software's specification should be able to affect the specification of the class).
A class should have only one reason to change. Separate GUI and the business logic. Low coupling, high cohesion.
Open/closed principle
Software entities should be open for extension, but closed for modification.
However, realistically you don't know which bits need to be extensible.
Liskov substitution principle
Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program. See also design by contract.
Bird class with fly method. Duck is a bird, seagull is a bird, ostrich isn't a bird because it doesn't implement fly.
Interface segregation principle
Many client-specific interfaces are better than one general-purpose interface.
Use small interfaces not fat ones.
Dependency inversion principle
One should "depend upon abstractions, not concretions."