Ahh, if it's Yasuo in particular, I can provide a few tips.
- Be sure to actually foliow up on harass. I haven't seen you play so I don't know if this applies to you, but way way way too many times have I seen ranged AP mid laners walk up to Yasuo, AA him to pop his passive, fall back to disengage minions...and then do absolutely nothing until Yasuo's shield is back up.
Like seriously, why?
- Yasuo's early game Q's are costly for him. You might not think it's much, but a 5 second cool down for a melee champion in mid lane is basically the entire world to them for that amount of time. If he Q's a minion, that's a 5 second window of opportunity to harass him while he literally lacks tool to fight back at full strength. Abuse it.
- On a related note, Yasuo's early game Q's are tiny power spikes in and of themselves, especially Q3 for self explanatory reasons. You'll actually notice this a lot when you play against aggressive Yasuo players: Q the minion wave once, wait 5 seconds, Q the minion wave second time, wait 5 seconds, and as soon as those 5 seconds are up, they will use their E to dash through the minions and attack you. If you can count those 5 seconds and/or notice Yasuo's telegraphed patterns of attack (and I must insist, they are very telegraphed once you understand how this champion flows), you can time hard CC to completely negate Yasuo's engage potential or even turn it around.
- Wind Wall is on a 26 second Cooldown at rank 1. 26 seconds. That is basically an eternity in the early game. You want to play similar to the point I made about Yasuo's passive, but through abilities. If you have a cheapish spam ability (usually your Q), try your absolute best to bait Yasuo's wall out, and take advantage of that 26 second window of opportunity if he ever drops it on your throwaway ability.
- Yasuo actually can't properly dash to dash through multiple minions early game; he usually needs at least 3 ranks to be able to multi-dash smoothly. This is related to the point I made earlier about Yasuo's Q spikes: Find the telegraphs in Yasuo's pattern. There's a delay between his dashes, use that delay to figure out his angle of attack and work around it.
Also, there is this obnoxious ass trick to negate Yasuo's E->Q combo. Buy boots, maybe tier 2 boots for the slowest of mid laners, and when Yasuo dashes up to you, walk into him. I am not even kidding, depending on where you are positioned relative to the minion wave, Yasuo will actually overshoot if you are walking in his direction while he tries to E->Q.