Eagle Vision
First complete the game. When the Memory Blocks are played again in Story mode you can kill anyone without penalties. Additionally, you will be able to use Eagle Vision at any time regardless of Synchronization levels inside of the Animus.

Killing Without Penalty
After you beat the Game, when you go back to the story mode you can kill anyone without losing life.

Unlocking doors
When you go to bed after Memory Block 2, you will notice your closet door is open. Hit any button to get a piece of paper Desmond notes as a code. The code will unlock your bedroom door which will allow you to be able to go out as often as you like after you are put to bed, as well as to open the meeting room area. You can also pick up various items to unlock Lucy's password.

Easier interrogation
After you get the "target" to a safe area to "interrogate" them, punch them first then "grab" them for faster completions. It will take about two grabs to quickly finish them. However, into the last three quarters of the game you may have to grab them up to, but exceeding, four times.

Easy fights
Only attempt this technique if they are fluent in the Rank 2 skill "Counter" and they have enough synchronization bars (a.k.a., life bars). One and a half rows of bars will do. When in battle, equip your hidden blade. With this equipped, all of your counters will score an instant kill. You can even instantly kill tougher ranked guards (for example, the Teutonic guards in Acre when hunting Sebrand). Also, most of the time using the hidden blade in battle will scare the last one or two (even three if you are lucky) guards, causing them to run away. However because you do not have a weapon that is sufficient enough to block a full sized enemy blade, all incoming attacks will hurt you.

Escaping from guards
Go to a high roof when chased. Kill the guards that appear. After a few kills they will either run or give up.

Defeating William of Montferrat in Acre
After the guards (or most of them) leave William, drop down from the castle and slay William. Afterward, "de-target" or deselect, and run up into the castle buildings and battlement to get away. This is the fastest way of completing the mission.

Defeating the Master
When fighting the Master he will take away your ability to use radar and make it difficult to see. This also allows him to hit you once and kill you. Lock on to him and use a throwing knife against him. This will take away his power to blind you and make him easier to kill.

Transformers
Travel to the southwestern-most vigilantes in the poor district of Damascus. On one of the buildings around them, you should see an outcropping in the wall, climb on it and you will see that it looks exactly like G1 Optimus Prime in vehicle mode. Note that there may be more than one outcropping in the area, so look carefully. If you lose sight of the vigilantes, you have gone too far.