that's a pretty trivial thing to laugh at you about =P
sheik's moves are faster than yours...so you pretty much have to be doing yours first. that basically means you have to play really patiently (read: campy). also try crouching. crouching works SO well on sheiks that don't really know the matchup.
as for samus...i dunno, be more patient there too. samus isn't hard for puff. sounds like you might be rushing into things. sidestep definitely shouldn't be working that well =P use shield a lot, samus isn't fast.
more generally:
sheik - crouch a lot. don't approach too hastily. played well on both sides, this ends up being mostly about spacing aerials, and bull**** crouching powers =P
- upairs and uptilts to rests, and sheik can't kill you too easily if you miss/they die off the side.
- edgeguarding: grab edge, sheik up-b's on to the stage, you rest her in the lag. if you're not confident about hitting that (it is kind of weird) then bair.
samus - use bair a lot. she can't get past it very well. most of your aerials beat her missiles. if you play really slowly and boringly samus has a lot of trouble here. just think of it from the samus' perspective.
you're samus...and this little ball keeps using a move with lots of range and priority, then floating far away from you...
- if she dash attacks you, shield it, then jump straight up and rest. do the same thing but jump towards her as necessary when her dsmash hits your shield, and i think fsmash if it's close enough. supposedly you can do this with a bunch of samus' moves, but i don't have a full list or anything. the dash attack one is just the easiest to get.
yeah, these aren't the most exciting matchups. with sheik i find that a lot of the time i have to make stuff up as i go, to trick the better character...but vs. samus i feel very safe using my various silly aerials.