In the majority of cases, I'm quite sure, that to live the longest, you want DI (Push the stick) in the direction, perpendicular, to the trajectory the move would hit you if your stick was neutral. The majority of moves that send u flying to the horizontal kill zones have the default trajectory (Non DI'd) of up left [for left killzone] and up right [for right killzone], so on the hitstun suffered from the move in question, you want to move your analog stick in the perpindicular direction:
So You DI:
"Up Right" if hit "To the Left side"
"Up Left" if hit "To the Right side"
"Left OR Right" if hit "To the Top"
The intention is to be hit toward the diagonals of the screen, (I think) You will always fly the same distance, but because your traveling on a harsh diagonal axis, your further away from the kill zones, because as we all know, the diagonals of a square/rectangle are much longer than either straight side.
After this DI'ing you want to Momentum Cancel:
In the case of Kirby, after Testing some, Gonzo does appear to be right about using Dair, for some reason, even though it lasts a little longer than Bair/Uair, It seems to be a better option as: it pulls your trajectory down, while in midflight, but you go the same/similar horizontal distance as if u were to uair/bair.
WHICH MEANS: You end up the same distance from the horizontal killzone, but are lower down, so your jump performed (immediately) after the momentum cancel has a far less chance of killing you off the top and off the sides.
Note:
-Using Down B seems to push kirby further away/ Keep same momentum until he's in a block form and falling... so not useful at all afaik.
-Using Over B immediately after momentum cancelling aerial pushes you back further than you would go if you didn't (Bad), and if used after jump... which is used after momentum cancel, seems to cancel vertical momentum and doesnt seem todo much to horizontal momentum.
-Make sure you push toward the stage after you jump... after you momentum cancel, because it stops some of your horizontal momentum, and can definately prevent you from dying where u would have.
Sorry if this is very incoherent, incorrect or inappropriate for this topic. I'm not sure if all of this is 100% correct, feel free to correct me.