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.