how to test void methods at junit in order to increase code coverage

I want to increase code coverage in junit testing . I have too many methods and all these methods are nearly in the type of "void". Therefore, I could not make assertian or any other junit tests in the void methods. I heard in the forums to test a variable in the void method to increase code coverage. But still it does not work ? What do i need to do to increase code coverage at Junit ?

If you want, I can provide sample codes.

Regards Alper