If you're still at low percents and on Dreamland, you can DI down and try to land on a platform.
Otherwise,
*DI'ing down will allow for more uair hits which will surely lead to an upb kill, especially if the Falcon can fast fall uair.
*DI'ing up just means the Falcon can do the upB finisher faster, but at least it's not always a sure kill since you'll have less uair hits on you and therefore a generally lower % (and therefore less knockback).
*DI'ing away means the Falcon will follow you until you go over the stage in which case he'll upB or dair.
*DI'ing towards is most likely the safest bet if you're to DI in a single direction as it's somewhat harder for a Falcon to follow you if you're DI'ing behind them.
I mix it up and DI all over the place, lol.
That forces the Falcon to constantly readjust it's combo string increasing the chances of an error which you can use to attempt a counter or trade (anything to get them to back off a bit) or at the very least try to second jump/fall away. So @
MrMarbles
is right, the best way to usually go about it is by being as unpredictable as you can.
This isn't just for Ness users either, any character who doesn't have a quick way to escape a combo (such as Pika's upB and Yoshi's second jump armor) will likely end up in the same scenario when caught in a Falcon uair string.