Try saving your E on Cait, your Q on Vayne, and your Lucian E. They are all little dashes that can be used to help reposition yourself or potentially even dodge something like a Leona stun if you time right. On Vayne you can also condemn them away if they get to close, and on Cait try to set up a little wall of traps between you and them. If you are being chased the traps are extremely useful. Basically, don't be that guy who dashes TOWARD the enemy unless you are sure you can clean up the kill and get out. Always save it. Unless you are Tristana, then you get the reset. Personally I think she's still the safest of all ADCs having a rocket jump and an aoe knockback, but that's an unpopular opinion because she's not meta.
Another tip I would give, is, unfortunately, you have to anticipate that that Leona or that Wukong are going to try to jump on you. You can't just hope they use it on somebody else, you have to anticipate it coming so that you can know when to stay out of their range. Especially if you are Vayne lol, Vayne has a global aggro taunt. Not really, but she might as well because everyone knows what a fed Vayne can do. Her and Lucian don't have the best of range (aside from the culling) so they have to get closer to danger to do their damage and sometimes it's really just not even safe to approach the fight. You just have to use your best judgment on that.
In the event of Shaco, Evelynn, or Rengar, I'd stay close to the scariest looking member of your team, or at least your support. It usually discourages them jumping on you unless they are hella fed. Watch the enemy's movements, if they suddenly look like they want to switch on the aggression it's probably safe to assume you're about the get flanked. Wards never hurt either to help you from getting flanked in the backline. Nevermind the camo champs, sometimes there's a Riven or Irelia sneaking up behind you from the jungle. I guess that's where map awareness comes into play, try to be aware so your backline (you) doesn't get erased in a fight. Don't feel bad about being a little bitch and running away, odds are you'll be level 12 when they are like level 15-16 so you won't stand a chance anyway.
This is why I split push as ADC a lot, my teams sometimes hate me for it but I just put it to them like this. Look, I can join you in that fight which I will be of absolutely no use in, and we might by some miracle win, or I can go bot and catch 4 or 5 minion waves and possibly even take a turret. That way, I'm actually working toward getting gold and closing that experience gap so that I'll become more relevant sooner. Usually splitting like that will either pull their ADC bottom, because he wants to defend his turret and catch the wave I pushed up, or they'll send an assassin to try and kill me. If I even suspect that's what going on I immediately step into a bush and recall because if I didn't go to the fight, pushed up and then died then my team would have a REAL reason to flame me. Lol. But my point is that sometimes fighting is just not a good option and instead getting more items and experience first is necessary.
One more thing, in a fight, just focus whoever is closest to you if you can't get to the squishies. Sucks if it's a Leona or Maokai or some other tanky mofo, but you just can't focus the enemy backline when you have a tanky wall of cc in front of you. Try to take their front line out first if you have to, fuck all those people who say don't focus the tanks.
This item is also your friend if the enemy team has a lot of burst and very little defense.
