The answer to improve on your smash is to practice, rx practice and practice. 80 to 90% of the power for a smash comes from your wrist and not from your arm or shoulder. The wrist must be used often so that you can feel it.
Used an empty shuttle box and flatten it a little. Sit down in the chair so that nothing else can be used for this simple exercise.
This will force you to use only your wrist so that you can train the power for your smash and improve it as well.
Put you left foot slightly in front of your right foot and then get yourself in a ready position to strike.
Ask your friend to hold onto something in a higher position and your job is just to slap onto the object by using your wrist. You can try using your arm and you can feel that the wrist will generate more power instead using the arm.
You can also practice this if you have a mirror or window in front of you to so the shadow of a smash. Try it out!