Watched you vs XPilot
Your movement is actually fairly good. Some flubs that need to be ironed out, but you transition between dash and WD pretty cleanly most of the time. It's good.
Sheik doesn't do shield pressure strings like characters in traditional fighters or space animals, Peach, Yoshi, etc. She's really dependent on having a bit of distance between herself and the opponent to protect her from some immediate OOS options (grab, aerials, etc). I see you go for shield pressure strings like [f-tilt > jab > d-smash] at close range - this is very unsafe. Grab, move back and space your stuff, etc. Down smash is generally poor on shields.
What the person above me said about rolling. You don't need to roll into people's space to try and make openings or get good positioning or whatever. You give up a lot of actions by doing so and lag yourself up a lot. Just WD back, in place, etc. and prepare to get ready for combat. Sheik's really good at combat.
You nair approach a reasonable amount. This is generally not very good vs Falcon and Marth. It's very easy for them to just dash dance around it and land a grab or whatever. It's also easy for them to beat it outright with a bunch of their moves.
You're also using nair to initiate a lot of shield pressure. But the thing is, if you're walking towards most characters and you see them put up shield, you don't really need to start shield pressuring like that. Sheik's best move to hit is grab. Grab them. If you want to do shield pressure because you're afraid of sidesteps or jumps OOS, space fairs or tilts. Nair doesn't really lead to much even if you hit it.
You also seem to like to dash attack to approach a lot. Sheik's dash attack is okay for this sort of thing, but it's easy to punish if they space further away (a lot of the time they're not even in dash attack's 'reach' when you're using it) and it can be countered on prediction if they are in its reach (via jump or shield). So, yeah, if you're gonna play that aggressively you should probably keep that in mind so you can plan counters and when you attack a bit better.
http://www.youtube.com/watch?v=Tuoljx3dfR0#t=1m32s
This little section was probably the best I saw you play. You could have gotten punished for that first up tilt but after that, you handled him pretty decently. Be careful about tilting under platforms like that. It worked out because he tried to drop through, but be wary of him stomping through the platform (crouch > fair counters this).
Up tilt and up air in your combos more. You can extend these by a few more hits. If you see a chance to dair them over about 40%, I recommend going for it. Dair leads to really good stuff. A lot of the time, you're also allowing the opponent to tech when you could just combo them directly. So try to get a better feel for your combo percents, trajectories, etc. You don't need to do all the work you're doing at some of these spots.
Don't try to predict them jumping over you by sticking up airs in their path unless they're committed in some way (already airborne, etc). Doing that against a grounded opponent is asking for trouble. In general, bair is easier to hit on people in free falls and cuts off landing areas or air pathways more easily (bair into f-tilt or more bair covers a lot of stuff). Uair is just a better combo starter in a general sense. But because Sheik's got low aerial mobility, a lot of the time you can't start combos off those kinds of uairs (especially at high percent). So just something to keep in mind.
I think you shield unnecessarily a lot. Don't be so scared of Falcon. Crouching and movement are a better defense against him in general anyway.
