Get Stout Shield instead of Quelling Blade if you expect to have a tough lane, or random and get both.
Get Phantom Strike first for escape/chase.
Max Stifling Dagger like usual but get 1 Point in Blur early.
Max Phantom Strike next and Blur last.
Get Phase Boots or Power Treads, whatever you prefer (I prefer Phase Boots).
Get Vladmir's Offering, then get Black King Bar, starting with the Ogre Club for more HP.
Typicall extension is Desolator for killing Towers and Skull Basher for lockdown. You want to end the game sooner than later, no rice farming.
Get a luxury or situational item if someone else in your team already has Vladmir's Offering or Desolator.
Get Armlet of Mordiggian or Drum of Endurance when doing bad early and you are not likely to finish a Black King Bar anytime soon (I prefer Drum).
Get Monkey King Bar to counter enemy evasion.
Get Manta Style if it can counter enemy lockdown spells.
Get Orb of Venom if you need even more slow early on.
Link to Purge's Coaching video: http://www.youtube.com/watch?v=0IOH__uNw_4
Phantom Assassin is really strong in the early midgame. If you farm an early Battlefury, you can't fight properly in mid game. Chances are good that the enemy outcarries you - if a Phantom Assassin and an Anti Mage get Battlefury at the same time, the Anti Mage is typically going to make better use of it and will generate more gold from it.