This combo is interesting, and works extremely effectively on big characters, and pretty well on small ones until the person you're fighting realizes they can DI out of it.
Anyway, for bigger characters, you can continue this combo about 5-6 times in a row, unless they're good at DI.
D-Smash > D-Smash > Up-B > D-Smash > D-Smash > Up-B > D-Smash > D-Smash > Up-B > D-Smash > D-Smash > Up-B > Down Tilt > Up-air.
That entire combo is for bigger characters like Ike, Bowser, Dedede, etc. I may have exaggerated how many times you can repeat it over, but SOMETIMES you may get lucky.
For smaller characters like Kirby, Pikachu, and Olimar, you may want to try:
D-Smash > D-Smash > Up-B > D-Smash > D-Smash > Up-B > Down Tilt > U-air > U-air
Because of ZSS's Up-special effect of pulling your opponent downwards, it may lead into the down-smash extremely smoothly.
Usually, this combo works for any character, but best at low percentages.
Charged or Uncharged neutral B > Dash attack (Best used when you're partially touching your opponent) > Up-tilt > U-air > U-air.
I've done this before, but sometimes you'll only be able to land U-air. Also, you may want to try Up-B after knocking your opponents into the air with the U-tilt.