I have some thoughts on movement improvement that I would like to share as well! Let's begin with the point/goal of movement!
Melee is a game where, for the most part, your character should always be doing something. Seldom should you be waiting and just standing. If we wanna get philosophical about it, we could be like, given that some positions are better than others, there exists a position(s) which is the best position.
So if you're position is not currently optimal, then you should change that, and you're wasting time (frames, really), not improving your position.
So to me, when working on movement, I wouldn't just like, drill l-cancels on the 20xx pack and see how many times I can avoid the red flashing in a row. That's a good start, but I would drill inputs after the l-cancel, and try to get actions out on the fastest frame possible. Examples of what I do to practice:
SHFFL fair> dash back
SHFFL fair> dash forward (both of these can allow you to avoid being shield grabbed, or lead to a follow-up)
SHFFL fair> SHFFL fair (defensive/spacing tool)
SHFFL fair> jab (frame trap that can lead to follow-ups after the jab)
Run> WD back> Dash forward (useful/safe option)
SHFFL bair> turn around ftilt (another frame trap)
Also, you should check out KK's movement drills. They are pretty sick, and are actually pretty hard:
http://myneverendingbrainstorm.blogspot.com/2014/10/diy-ssbm-movement-drills-part-12.html