I'm stealing this post from another thread, because I'd like to bookmark it directly, and I don't know of a way of directly linking to posts, rather than threads 
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Hello
I main ADC, find that it's the most fun and rewarding role to play, save maybe Jungle.
Crucial skills to have:
-Ability to last hit effectively, especially under turret, and controlling minion waves
-Managing your mana pool effectively
-Knowing your damage output, your ability to take damage before it becomes unsafe to be around
-Sense of intuition and map awareness
-Location, location, location (aka Positioning)
The easiest champions I found to brush up on last hitting and positioning are:
Vayne (You learn out of absolute necessity, and you learn fast or crumble)
Corki (great repositioning on Valkyrie! Excellent base AS for farming + true damage passive)
Last hitting is easiest when you have a support that is good at zoning the enemy. Lulu and Nunu are highly effective at this with their amazing harass. This allows you to farm comfortably without worry of needing to harass on your own. For video examples, I suggest Doublelift's Razer Academy video for Vayne.
http://youtu.be/NPGnF7vJ4Sc
As far as managing your mana pool goes, getting familiar with your champion is really the only way. It can be combined with map awareness skills and intuition, which I'll get to next. You don't want to leave yourself with a small mana pool when you need to use a skill to escape. This includes flash.
Map Awareness, Intuition
Map awareness can be defined as knowing where everyone is on the map. This can include what state they're in, both their HP pool and what items they have bought. If there is an extremely fed Lee Sin running around on the enemy team, at high health, and you don't have vision of entry points into your lane, you probably don't want to be pushing waves of minions to the enemy turret.
However, if you know their jungler is in top lane, their mid is in their lane, you can feel free to turn up the aggression for a bit.
Intuition goes hand in hand with this. You may have wards, their mid/jungle hasn't been seen for a minute after backing or had a decent amount of health last time they were seen, you're doing pretty well in your lane so you're pushing the creeps up and denying your opponents cs to the turret. You're probably up for a gank. Play a bit more cautiously, make sure that you have enough mana for an escape skill or that your flash is up. If you have neither, go ahead and back, grab items if you have the money, do your thing.
Talking about the game is making me want to play the game, I've been on the forums too long and this post alone I've spent at least 20 minutes on, lol. Hit me up in-game if you want a bit of hands-on practice. IGN AvariceSyn
Edit: Oh god, oh god, oh god, see posts saying servers down. NOOOOO!!!
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TL;DR:
Last Hitting/Vayne Guide: http://youtu.be/NPGnF7vJ4Sc
Freezing Lane: http://www.youtube.com/watch?v=xh0Oa-Llu58
Positioning: http://youtu.be/8NxlLE5S34Q
^ a bit slow, but highly informative
Kiting: http://youtu.be/On-7QLVY374
Same creator as Positioning video, bit more lively
This guy plays a lot of ADC, I recommend watching his other videos as well.
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