What if you're up against an obviously duoqueueing Xin Zhao/Vayne and they're harassing the hell out of you and your lanemate, who's really too afraid to do anything? We got out-harassed and ended up feeding the Vayne/Xin Zhao far too early. Eventually, the only thing I could do was dash in, stun the damn Xin Zhao before he got a slow off, slash all of the minions, and dash out. My lane-mate didn't even try to get any last-hits.
Needless to say, we lost.
It depends on which champions you were using, but generally speaking:
If you have a jungler:
Call him, beg for a gank. At the very least, your gank should soften your opponents up and make them wary of your harass. A Xin Zhao with low life can't use his passive to farm your minions for HP, or else you'll nuke him down.
If you don't have a jungler:
Hug your tower and learn to last hit creeps that the tower has softened up. If it takes you two tower hits + two autoattacks to kill a melee minion, make sure you hit each one appropriately. Pick your best farming skills and use them generously. You won't be able to last hit everything, that's the price of hugging the tower. Keep your stun/cc/silence/snare ready so Xin Zhao/Vayne will be wary of tower diving. Allow your tower to take some damage, but by no means have the lane abandoned at any time. You WILL lose the tower eventually, just try to squeeze as much good as you can out of it. If you're lucky they will overextend and your mid will sneak down to help you kill one or both of them.
Nope, no jungler. I'm not level 30 yet, so in many games there's pretty much no supports/jungler/wards. I only had a stun and my lane-mate was Master Yi, so really, there wasn't too much we could do. Even worse, Xin Zhao and Vayne refused to overextend. They would just sit within xp range when they had pushed all the way to the tower. Mid was otherwise preoccupied ganking top, who also needed help, just not as desperately as bottom.
And the problem with last-hitting tower-softened minions is that your own minions tend to over-soften the enemy minions, preventing effective last-hits. Cull the Meek is good, but not THAT good. I probably should've been spamming my abilities more often just to get those last-hits, regardless of the cooldowns.
Not that junglers are that effective anyway at level 15 or so. Half-full masteries and half-full (or completely empty!) rune pages probably have something to do with the junglers ending up far too underleveled by the time the mid-game starts.