Do NOT Shield too much against Wario. A lot of times Wario players will bait your shield by feigning an aerial approach, and bite you. Bite is an extremely good option because it pretty much renders your OoS options obsolete unless you catch him doing something like dairing your shield, which many times they'll move away.
Staying under him, like CK said, is also really good because it's the opposite of where you don't want to be, which is above Wario. Wario is insane at juggling/gimping Marth so even if you can't juggle him, try to keep on an even plane or under him.
Watch the timer. Even if you're like "WTF HE JUST **** ON ME" don't be so stunned that you can't take a quick look at the timer, because knowing when that next 2 mins is up will help you a lot.
If Wario dairs across your shield/FF it, it is, in fact a:
(Vote Admiral Ackbar '12)
He's going to do something after the dair and it will cover many of your options. Rolling away is one option, or jumping OoS, or if you're feeling bawlzy you can try to DS, but you might get punished. The 4 things you absolutely don't want to do are rolling behind, staying in your shield, dropping your shield(whether to do anything/just to drop it/trying to SG) and spotdodge. Bite beats many of these options and the others he can Fsmash. Keep in mind you should mix it up if he keeps doing this, though, because a good Wario can read you(i.e. chasing your roll, uairing you as you jump OoS)
If you get Wario off stage, harass him. Don't go above him, yet again and try and go for some dumb fancy spike off his bike, he can jump off the thing and punish your *** as well as make it back. You can bait him off his bike or just flatout hit him with something like a tipper bair. Keep in mind his recovery without the bike is HORRIBLE. If the bike is onstage, don't try and blow it up, try to keep him away from it. Gimping Wario without a bike is easier than gimping Ganon.
Oh and watch out for dthrow stuff. Some will fsmash, some will bite, some will do other things out of it, but try not to get predictable with what you do when he dthrows you, because like i said he has a lot of options in relation to what you can do.
Last thing, DON'T TRY AND DO NOTHING BUT GRAB! But keep in mind that you should be using grab to punish in this matchup more, especially at high percent or if your usmash is relatively fresh.