there are many ways to improve kicking.
one way (and its not all that *fun*) is using a half board with arms fully extended and face (freestyle snorkel) in the water.
now if your shoulders wont allow it you can always kick on your back arms out.
there is also the use of burners, full fins and the mono-fin.
btw, breaststroke kick is just an invention of knee docs and should NEVER be done. nor raced! (partly kidding).