I might add this to my FAQ...
Contrary to what some say, how hard the jump button is pressed has no effect on the height of jump. Instead, the player is given several frames (varying per character) to let go of the jump button before the jump command is executed to perform a short-hop. The frames for Kirby has been increased from the transition of SSBM to SSBB. It is possible to short-hop with an analog stick, although it is considerably harder.
So how to practice?
This is a skill of reflex/finger muscle memory. Go to training mode, press the jump button, and let go as fast as you can. Repeat this for about 10 minutes (since fatigue does not help memory) each day/session until you get good. It takes a bit of time (I know firsthand, especially with SSBM Kirby), but is definitely worth it.
After that try running, jumping for a short-hop, and then execute an aerial. This is harder, since you are no longer just concentrating on short-hops. This is a valuable skill, and takes practice.
I actually did something creative to learn: in school during my spare time, I turned my TI-83 calculator upside down (to benefit my right thumb) and started the Super Mario game. I had small Mario perform short-hops against a basic block in that game. Short-hopping is much harder on a calculator. Since the ergonomics of holding a calculator and pressing a button quickly is not comfortable, I just had my thumb reach upwards...as a result, now I press Y for my short-hops...
Good luck. When frustrated, do not think of failure. Think of how you will eventually short-hop whenever you want.
