There are many ways you can do this. One way would be to make objects fall at a speed of x + y, where x is whatever speed they fall at originally, and y is a variable that increments every time 20 lines have been cleared.
I'm not going to and other people should not. That is not the way to learn. If you post a link to your game code, then I can help you out a bit more, but copying + pasting answers is not the way to learn, after all, next time you come across this problem, how are you going to solve it?