fox has to be near you eventually...those two characters can really destroy each other, so i find mostly you need to get quick kills on him the way he will on you. so you can 1) use the hungrybox method, which essentially means throwing them off the stage and edgeguarding with aerials, or 2) combo into rest a lot. i find the latter more fun =) upthrow-rest of course works, but fox can be hard to grab, so you can also try mixing in upairs with the bairs and fairs, and i like using reverse nair to uptilt a lot (don't even have to L-cancel it)
oh, also with fox (i'm still working on this myself) i'm told when he upthrows you can smash DI out of the strong hit of upair. one way is to dash dance as he hits you and hope that works (supposedly kdj does that) or to DI one way from the upthrow, and then the other way when the upair connects, so that hopefully you'll be pushed out after the first hit.
vs. sheik...crouch a lot, hope she doesn't think to stop JCing her grabs. sheik has difficulty grabbing you when you're crouching, so if the opponent doesn't know how they probably won't be able to, and once she misses the grab you can grab her, or do moves, or just rest her in the lag. rest combos work well on her. and edgeguarding is important...if you can force her to up-b onto the stage, rest.
vs. marth...you have to be as patient as he is X_X try to bait things from him and just generally don't rush in. running up with shield works a lot, and if you can get them offstage, try to force them to up-b onstage so you can rest them or continue edgeguarding (thanks for that hbox, but most good marths won't recover onstage vs. a jigglypuff anyway).
tl:dr...marth - be slow and edgeguard well
sheik - crouch a lot, rest combos work, edgeguarding is important
fox - use all of jigglypuff's most broken tactics...don't camp though, that's boring
^ i don't suck, some of that is probably worth reading =P