Generally, I just sit in the minion wave like I just want to farm. If Yasuo comes to me, I back farther into my minion wave (since he's going to cut in with his QE combo) and activate W. The individual orb damage should chunk out his shield within the first two, meaning the third is a free hit and a refreshed shield. Auto-attack until he leaves and if he stays for three auto's, he's chunked and loses the trade for sure. Back off farther if he gets his knock up. I generally sit 9/21/0 for reference, so I don't have that power that a lot of people like, but it is really counter-assassin diana.
The idea with Diana early is just to make trading with her such a nuisance that she doesn't seem to be bothered. Cloth five to his Doran's means you out sustain him in one or two trades. If you force him to be the aggressor even though he prefers to be in your minion wave, he's going to stop because early game minion damage is actually really high.
Generally on one of my first backs, I go boots+dagger or boots+warden's mail if I have enough. Because it is a relatively escapeless lane for both if the minion wave is shoved, I'll buy the yellow trinket's vision upgrade as opposed to the stealth totem. That way, I can help my jungler get into lane and let Yasuo shove up a bit too far. Usually if you get a kill onto Yasuo, he's irrelevant for the remainder of the game to you because you'll be nearing your armor item before he gets shiv or IE.
To conserve mana for my shield, I generally don't poke Q much, even though I generally go 2/2/1 for QWE before lvl 6 (After, I max Q as normal). Instead of clearing waves like that, I just use the passive procs. If you get zoned out, I highly suggest immediately backing and forcing your jungler over. Yasuo has an amazing ability to dive turrets and Diana has so little ability to maneuver against the straight line AOE knock up of Yasuo's Q. Better to back for HP then die under turret and hand off the lead. Poking with Q for farm is such a bad position that it signals immediate intervention needed.
Once you hit your ult and get the primary upgrade to Nashor's tooth or whatever big damage item you go, going QWRER is not a bad trade. Just make sure you do so when his knock up is down, using the boots to help dodge. Technically at 6 if you're even before, its your win if you dodge the knock up. You gain so much range at 6 that Yasuo TECHNICALLY has, but shouldn't be able to use if you have boots and some dancing moves.