Online and offline play are different, so l2p.
^l2p lol
I've practiced almost solely off of Wifi and training mode. I'm currently trying to get matchup experience from wifi play. But, if you do so, keep in mind that online and offline play are different. Different mindset, and at times, different playstyles and techniques.
For example, I play Sonic. Online. His offline game is heavy on attack-lag punishment via shieldgrabs and spindash combos. Since, through input lag, all my moves come out later, if I try to punish that missed tilt, I'm going to be running into a smash or a spotdodge-smash instead of getting a shieldgrab. I lose a staple part of my game. So I keep that in mind and use it much less, unless I can predict my opponent doing a counter attack and just do a run>shield (Marths tend to use their fsmash, so I can space the shield for that case).
At the same time, I have to buffer my spindash combos ahead of time and hope that I lead the jump to aerial correctly. And you know what, I've done it, and it's possible. If I'm playing online against someone who's trying to get matchup experience, I point out to them anything that wouldn't have worked offline, and their vulnerabilities.
While offline play is very reflex-based, online play is more prediction-based. Predict your enemy's moves and buffer in a counter moveset ahead of time. "Predict" your timing and do your move inputs ahead of time, without having to watch your character (especially for things like getting FH-FF or SH-FF aerials to hit near ground level).
Also, read this, it might help @_@
http://www.smashboards.com/showthread.php?p=4758359#post4758359
Offline, practice your timing for shields and any 'reflexive' moves, like punishing smashes or timing-tight moves like chaingrabs and spindash combos (all of which I've seen done flawlessly and multiple times THROUGH LAG online). Online, practice predicting your opponent's moves and finding counter moves to them, while keeping yourself unpredictable. Mindgames and all. Their "reflexive" moves and counters are done ahead of time, and that should say something about YOUR predictability. Just keep in mind the differences, especially if you're up against someone with alot of experience online.
[edit]
On a random note about predictability and people doing moves as if they were reflexively done offline:
one guy I played on aib, samboner, kept grabbing me out of a half charged spindash online.
...through .4-.5 second lag.
wtf.