Complicated tip:
You guys should all be shaking out of stun during free-falls (or instant needle store to get out or something of its ilk). As part of Sheik's nature as a character, if you're falling down against Fox and beat his move with yours as he's trying to continue or extend his combo, unless he's at super low percent he's probably dropped his combo. Because he can't always beat your move via positioning because of her range, his counter to this is to commonly wait for Sheik to throw something out and then punish the lag because, what with you falling from the combo, you're probably not spaced & timing your aerials perfectly for low approaches (actually this is true for most characters vs Sheik).
This dependence on him waiting for lag & punishing it massively empowers wavelanding away as a combo break & pressure escape tactic in free-fall situations. However, you need to get yourself out of tumble immediately and WITHOUT SPENDING YOUR JUMP. Wasting your jump is the easiest way to die against this character because then you don't have it available as a mixup (DJ > waveland or attack or whatever is very strong) and it basically makes it so that Fox needs an up tilt to kill you because then he can just horizontal juggle you until you're offstage and you don't have the aerial mobility to escape something like that without the presence of that mixup.
Air-dodging away from characters to get baby wavelands while still retaining some dodge invincibility is really good against certain characters in these sorts of situations (Peach). Add it to your repertoire. It's a very strong mixup. Don't be lazy; don't tumble when you don't need to.
Significantly less complicated tip:
If you DI away from Fox's up smash at any percent (including 0%) there isn't enough stun between you being up smashed and landing on the ground floor for you to legitimately be forced to tech. But many of you still get up smashed, fall on the ground, and then tech. You don't need to do this. Shake, double jump, needle to make your autocancel (not sure about this one; haven't tested it yet but I think it would work the same as Fox & Falco's laser thing versus Samus) are all much better options because being forced to tech sucks even if you're Sheik.