I like the sneak in through the basement, go up the stairs and out on the ledge route myself: many thanks to the silent assassin who first posted it for the rest of us to enjoy. Quick and dirty: take out the incontinent guard with a knife, take his clothes, and run, yes run, to the cellar. All the guards you'll pass are facing away and minding their own business. You can also run through the kitchen, though you might alarm the crossing guard there if he happens to pass through the same time as you. Up the stairs, out on the ledge and around to the Don's office. Only problem here is you may have to wait awhile since you made such good time. Crouch and lurk outside the west door: monitor the map, keyhole or the white crosshair (it'll turn red) for the Don's presence. Tip toe in, knife him, and make your exit through the east door. Take the ledge and drop right into the cellar steps. Run in, run out. 4:20 last time I did it.
You can also just sneak past the pisser at the door and do the mission in your sunday best, but then you have to be careful in the kitchen: the chef will freak if she sees you; but if you run up the stairs and onto the ledge without a guard seeing you you're good to go. For added excitement take the axe, golf club or katana sword. I'm a little disappointed that none of them were good for one-whack kills (a 10" ginsu knife more effective than a 3,000 year old Samurai sword?), but you can still get SA, and it is a hoot to see "the Don is alarmed" message after you take the first whack. "The Don is alarmed?" Yeah, he's alarmed: you just whacked him once with an axe/driver/sword and you're setting up to do him again! Fortunately you always get the kill without any alerts, though sometimes his lawyer makes a mad dash into his office, so you have to grab the key and scoot. First time I did it I actually bumped into the lawyer on the way out: I was in disguise and standing near the body, but the computer didn't penalize me. Might have been the axe that fooled him.