To lower the time needed to complete the game and 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.
Successfully 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).
Once all of these are completed, go back to the Assassin's Guild to start the major
assassination mission.