Attackers: Don't forget the bar area! Many defenders don't expect you to take this route, and thus leave it lightly guarded.
Defenders: The bell tower can be an effective sniping area, but use this location with caution! If your team is suddenly pushed back, you may become trapped up there. Also, be aware of people climbing the rope; attackers love to sneak up and stab you in the back if you aren't careful.
While it is easier to plant the bomb near the Drugs, this area is also easier for defenders to take, allowing them to defuse the bomb and win. Try planting at the Truck occasionally; there are many hiding places and balconies from which you can attack bomb defusers, and it is less confined.
A well-timed LAW or other explosive in the courtyard can take out an entire team.
Take advantage of those second-story hallways! Many lead to very nice sniping positions.