Practice for hours, HOURS! I have over 20 hours of playtime purely in training mode. Literally sit in the training room for 3 hours in a row every day with 1 (ONE) team and figure out how each character works inside out. Figure out how to maximize EACH move. Figure out which moves are easy to land, which are safe to use, how to best approach, what is your maximum damage potential. Master your long combos so that you'll always do them correctly. (look them up on youtube if you don't know them now)
There's countless little things like that, it's called "depth" and it's why fighting games rock.
Btw, the AI even on "very hard" sucks. It may read your button presses and react but it never adjusts and it's extremely predictable. Only against human foes can you truly test your metal.
If you're on PS3 I could show you a few tricks sometime, my online record is something like 700-85 btw, I'm nowhere near unbeatable but most online people who play this game currently do suck a ton

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