Beginners' Guide to Assassinations
This guide is directed at those who are still playing through Assassin's Creed for the first time. The greatest joy in the game comes from figuring out how to stealth assassinate one of the main targets on your first try, without reading a walkthrough first. For me, this satisfaction came much too late (the eighth target). I was good at the "not cheating" part but lousy at planning an assassination, mostly because there were aspects of the game play which I did not understand.
What follows is what I wish someone would have told me before my first assassination attempt. There are no spoilers (I hope), just general advice on planning, executing 
, and escaping from an assassination.
Fighting
To be honest, the game becomes too easy once you figure out the basics of swordplay. You'll basically never die again (unless you fall into a small puddle of water 
). It may prolong your enjoyment of the game to figure it out for yourself. If you're frustrated because combat is too hard or if you feel that you've already mastered it, then Assassin's Creed Gamer has the perfect guide for you.
Hidden Blade
Learn the basic Low Profile and High Profile assassinations with the Hidden Blade. Low Profile is easy. Just walk up to a guard and press the X Button (Square on PlayStation 3). Also practice a running, High Profile attack with the Hidden Blade. Starting at a distance from a guard, hold the Right Trigger (R2 on PS3) and run towards your target. When you get within a few meters, continue holding Right Trigger and press X (or Square) to jump on him with the blade. This technique will come in handy if you ever spook one of your main targets and he tries to run away from you. You can't Sprint after someone with your Sword drawn but you can run them down from behind with the Hidden Blade.
Investigations
Do all of the investigations, not just the minimum 2 or 3. Sometimes the information gained is really helpful in planning your assassination. Review all the information by pressing Start, selecting "Memory Log", and cycling through the investigations with Left and Right Trigger. There will always be a "note to self" from Altaïr interpreting the information and how he can use it. Pickpocket and Informer missions often have an Attachment you can view. Maps are particularly useful.
Exploration
Figure out where the assassination will take place and explore it before returning to the Assassin's Bureau. Your investigations will reveal the exact location, which will always be in the same District and will generally be within a large landmark. Plan an escape route. Save some Citizens near this site and on the way back to the Assassin's Bureau. Doing so will set up Scholars to help you in and Vigilantes to help you escape later.
Approach
Sometimes the assassination area will be guarded. There will be more than one way to get past the guards. Scholars are always a safe choice. Don't get nervous about botching the assassination at this point. Every target makes a big speech (cutscene) before you have a chance to kill him.
Assassination
After the cutscene you want to remain Anonymous and eventually take down the target with your Hidden Blade. Minor spoiler:
).
Escape
A good plan is to Sprint towards the Assassin's Bureau, passing through one or more groups of Vigilantes and then climbing onto the rooftops to find a Roof Garden hiding spot. The Vigilantes will grab your pursuers and make it easier for you to break line of sight. You can check your map as often as you like during the escape. Place your user marker anywhere on the map (not just on an icon) to help find Vigilantes or the Bureau.
A little tip for interrogation missions, if there is a ladder near by then draw the target there. That way after you kill him you can make a quick get away.