well i probably am the 2nd best in FL but i'm still leagues behind plup lol
mcninja, i'm assuming you mean tech skill like...fox being able to nair shine, and wavedash out of shine, and stuff like that?
in that case...let me think...
• jab cancels - basically you can cancel the window for your second jab (the canon arm swing) by crouching, or walking (i think walking works...i think). example vid:
http://www.youtube.com/watch?v=orIRuYN6hP8#t=2m59s afaik plup uses the jab, crouch, jab method. the other popular one is there is a specific angle that you can acheive w/ the joystick to allow for easy jab cancels, let's say you're facing east, i think the angle is just above the 45 degree south west, holding this should allow you to do your first jab much faster than just jab, wait, jab. if you miss the angle then you'll either crouch and then Dtilt, or ftilt west. something to note about this technique, you're goal is not to hit them with a jab, it's to get them to do something stupid either out of shield, or after getting hit by the jab. in the example vid, plup jabs him and he doesn't know what to do, he drops shield to wavedash or jump, idk, gets caught by a jab, and then plup dsmashes him while he's flustered. some other popular options out of this are to roll, spot dodge then CC, or shield drop cc ur jab punish, and some others. basically while ur jabbing ur anticipating what they might do and readying yourself to punish it. if they get hit by the jab, how are they going to react? can u do as plup did and dsmash? or are they gonna shield it again right away so maybe u should grab.
• missile cancelling on all the stages, yoshi's and stadium you only need to short hop, but for battle field and dreamland you need to jump, then double jump quickly after that. you can either tap the jump button quickly, or you can button slide like i do (slide my finger accross y then x, akward at first but to me it's normal now), or you can use maybe the joystick then a jump button or vise versa. they all work, and they all have their flaws, main thing is to pick one and stick to it, don't be switching around or doubting that you can be consistent with a certain method, they will all work.
•pivots are quite good, pivot fsmash/ftilt (i haven't been able to get consistent with Ftilt to the point of using it on command, but usually when i pivot either ftilt or fsmash comes out).
•UB OoS, and subsequentially learning the shieldhitstun for every attack in the game, it sounds daunting but only a few do you need to be really specific on. basically you do not want to mess this up, or not be able to do it on command every single time. basic samus stuff you should not mess up on. also, UBing when you UB, attempt to ledge cancel it EVERY TIME while you're learning it, there are instances where it's better to fast fall and land, but in the vast majority of matchups ledge cancelling it is boss.
••also in this category is wavedashing OoS, samus has a good long wavedash, make sure you can do it as long as possible out of shield, as well as being able to control the distance you wavedash
•sweet spotting your UB is also pretty important, i'm pretty sure you can sweet spot under marths fsmash and dtilt, so learn this. to practice you can go to training mode and lay a proximity mine on the very very edge of the stage, (but actually horizontally on the stage, not vertically on the edge), get it as far over as u can, personally i will roll toward the edge, jump straight up, then drop it down, that should get it as close as u need. from here just practice sweet spotting with your UB, as well as getting into the habit of holding toward the stage, and pressing R/L to tech at the right time. if you sweet spot you should not get hit, if you don't, then your automatic smash di + proper R/L timing should cause you to edge tech.
as a whole though, you need to not mess up, with whatever you decide to do. that is the most important part, and that defines good tech skill. you would rather be the fox who can nair shine his opponents accross the stage 100% of the time than the fox who can multishine ~ 10 times in a row on a good day but messes up simple bread and butter combo's because he spent his time practicing multishining.
also, refine your edgeguards, uptilt is great, it has a very low reach, i saw you were using bombs, i went through a bomb phase and still use them for sure, though a little different than you, but honestly you can get consistent with utilts to the point where the bomb actually prevents edgeguards some times (ie, the utilt hits, then the bomb hits, and u can't follow up)
so i would advocate using down angled ftilt, fsmash, and utilt to edgeguard almost exclusively instead of bombs, because you can always fall back on bombs they're not hard to lay down where you want them, but learning the timing & spacing for utilting/ftilting a marth sweet spot UB is a lot harder.
some advanced stuff to learn
•aerial interupting, it's quite good
•full hop double missile, situational, but can wreck people who aren't used to it
and keep in mind...just because you can do a technique, doesn't mean you've mastered it. the higher level you get the less mistakes you can afford to make. i've see matches loss because of a failed wavedash, it is a big deal, and ialthough it's tedious and not fun to practice, it's very important that you be consistent with every part of your game