Movement. Plug in a second controller so you can get an opponent that doesn't move/attack (though a level 1 CPU doesn't do all that much anyway) and just practice moving. If you ever drop L-Cancels, work on those, and try them both whiffing the attack and hitting the attack, as the timing is different.
I like to try and do something 10 times in a row without messing up. If I do, I stop right there, go do something else, and come back later to see if I can do it again in less attempts. It's a good way of building muscle memory IMO.
Don't put in too much offline practice time, I do 15 mins a day unless a tournament is coming up. It's not worth the time unless you have a real thing you need to learn.