Sheik's fair autocancels very easily. Basically anytime before the apex of her short hop is easy, and even at the apex is fine if you just fastfall a split second later.
I find retreating or spaced fairs to be incredibly important. For example, you can practice sh fairing so that falcon or ganon can't shield grab (by being just far enough away) so that you can just dtilt or ftilt them after. Retreating fairs i do on approach so that sheik moves her body away and then slaps after their attack. This works surprisingly well on fox's upsmash: you can dodge the entire hitbox and then slap him.
Against characters like marth you can practice fairing past them, which is slightly trickier and requires you to be close. This is useful because marth's grab range *****, but can't grab you from behind as well.
Also my biggest problem with both my mains is dashing as soon as the lag ends from my attacks.
I feel like understanding sheik's attacks is important..
-For example, Sheik's bair I see as the most often misused attack. a sh rising bair can only hit someone on the ground with the weak part, but it will go through fox's shffl nair approach. A falling bair is dangerous against a lot of characters that have high priority attacks that can hit between sheik's legs, such as peach's dash attack or fox's upsmash. So against these characters, it is imperative to throw out the bair later so that they can't catch your leg as you fall. If they're in the air, rising bair works pretty well. Also, the weak part of bair is dangerously easy to cc.
-Weak bair can also be used in combos
http://www.youtube.com/watch?v=JE1O-m7Jt2c&t=1m43s
^ Rising reverse weak bair to falling fair
http://www.youtube.com/watch?v=2UngjR_bTTQ&t=3m1s
^ Weak bair to dash attack. You can see this is hard to tech because they've already hit tech anticipating landing before the bair... kinda situational.
http://www.youtube.com/watch?v=Iwi17ImItR4&t=5m8s
^ stupid combo that works on falcons...