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.
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."