Code reuse

1) K.I.S.S. : Keep It Simple, Stupid! The more it is simple, the easier it can be used elsewhere.

2) ONE and ONLY ONE feature by system. By system i mean one or more classes that work together and never should be used separately. Your system could be a String class (one class). Or a Task class and a TaskManager class (2 classes). The point is to make it like composants that provide one feature and then can be used in unknown ways. Think of it like electronic composants : each one provide only one specific feature.
When you have your ONE feature system, then you can make other ONE feature systems on top of it that enhance the previous system. This way, when you need only the original feature you can use the first system. When you need more advanced manipulations of this system, you can use the systems on top of it that provide the features you need.
Make it fine grain.

3) Make it Black Box : Don't assume the user will plunge in the guts of your code to see what should happen. Simply make all the usage of your system (THAT PROVIDE ONLY ONE FEATURE!!) obvious by looking at the provided interfaces. Isolate your system from external noise.