+ MF has great passive which allows her to harass opponent,dodge skill shots and escape from ganks( if you don't get hit in beginning, you should be able to run from Udyr and heroes like him ( without charge skills))
+ Double up + Normal attack deals ~ 400 damage at lvl 9 (5th rank of Double up) if you got a B.F. sword. Considering that as normal carry (mage or AD) they have around 1500 HP, that means you can poke them really hard or potentionally "zone" them.
+ Her ultimate combined with amumu's or galio's is very strong in team fights.
+ Her W is good when laning against heroes like Vladimir.

- She is of course very squishy and she doesn't have escape ability like Tristana or Caitlyn so you must be very careful when positioning in team fight, because you can die really quickly.
- MF without her passive is one of the slowest champions in the game, so be careful.

Exhaust + Flash = Good combination, With flash you can escape when focused in team fights. Exhaust is good in lane against other carries or solo laners. When you are in teamfight, Tanky dps's or tanks will most likely focus you, so you can simply exhaust them and run or take them down. Exhaust is also good when you have 1v1 fight with stunner, burster or strong melee against you.
Flash + Ghost = Another viable combination, replacing exhaust with ghost allows you to have a better movement around the map ( you can ghost in jungle and run to the lane, you will be there sooner then they'll expect ) ghost is on the other hand not that good in team fights ( when you are slowed, ghost doesn't basically do nothing ) but you can chase enemies with it. Ghost in 1v1 ( when not chasing ) is useless.

Boots: Berserker's Greaves or Mercury's Threads You should start with Berserker's Greaves if your opponent isn't someone like Malzahar (stun, strong 1v1). After some time ( ~20 mins in game) you can switch to Mercury's Treads because you already have a lot of attack speed and Mercuries are really good in teamfights ( because of unique passive - tenacity).o
Core Build: You should get those 2 items in every your game.
[/b] Infinite edge is just really great item, 80 AD, 25% crit and Unique passive +50% crit damage, basic item for lots of AD carries.
With phantom and Infinity you are pretty much unstoppable.

Another viable items:
Survivability and slow on hit, very good item.
Every AD needs some lifesteal.I recommend you to buy Vampiric Scepter early and finish Bloodthirster as 3-4th item.
Also great item, good against tanky dps (~100 armor.)
Good against very tanky teams and tanks(~150 armor) if more than 2 guys have that many armor, you should definitely get Last Whisper.
Good against high Hp team, it will also give you some armor so if you are dying from AD guys, you can buy this.
Good against mages, also give you nice attack speed so you can combine it with Madred's Bloodrazor
Survivability, absorb one spell every 45s and has decent amount of Hp, mana and MR, why not?
Your final build can look like this:
mercury's threads

Start with Doran's Blade. Last hit as much as you possibly can. If your opponent is good and you don't have opportunity to farm that well or if he constantly harras you, you should buy another Doran's Blade ( Your maximum number of Doran's Blades should be 3 but I recommend you to buy max. 2 of them) You should buy B.F. sword as soon as you can, but don't keep your money for too long. For example (it oftens happen to me) You return to base for the first time, you have ~1300 gold, what should you do, should you return to lane and get 350 more gold to buy B.F. sword? Well I think not so I will, instead of keeping my money, buy Berserker's Greaves and another Doran's Blade.
If you have lot of money on your first port home (~2500) you should buy Boots, Vampiric Scepter and [B.F. sword]]. If you get a lot of money on your second port, build Infinite Edge before completing boots.