To lower the time needed to complete the game and to decrease the difficulty of 
each major assassination, do the following in order (with the exception of Masyaf).

* Visit the Assassin's Guild.
* Scan the city by first climbing all of the view points.
* Rescue all the citizens from the thugs. You will need the help after a major 
  assassination. 
* Complete all of the informant's missions (to get all the valuable information 
  you can). 
* Pickpocket the shysters in all of the pickpocket missions (for information 
  to the Guild). 
* When all of these are completed, go back to the Assassin's Guild and get ready 
  for the major assassination mission.