One of the things which helped to get into OO mindset, along with the practices that earlier posts outlined has been, is to rewrite/improving the existing code you have written using OO principles.
For example :
a. In situations where there is a lot of if/else constructs then probably
you can think of having a class hierarchy to distribute the branch codes accordingly,
and to use polymorphism.
b. Any use of operators like (instanceof in Java) would indicate programming to
concrete types and you can think how the instanceof check can be get rid of.
c. Use "Law of Demeter" as a guideline and see whether the coupling between
classes is high
To an extent the practice of "Test Driven Development" also helped me
since it forces you to think in terms of interfaces/behavior to be exposed
by a class rather than just concentrating on how best solution to a problem
can be coded.
