What else?
Remember to not be a casual.
Summon up all the casual thoughts in your head.
Then do the opposite of all of them.
Alternatively, think "What would Autumn do?" then, do not do that thing.
**** your tiers
Tiers are very important. You can still win against an opponent with a higher tier character, or a match up that's not in your favor, but the odds are definitely not in your favor. For every one mistake you make, your opponent will have to make several for you to come out on top.
Of course it depends on the game. Marvel Vs Capcom tiers are vastly more punishing then, say, Guilty Gear or Vampire Savior tiers.