I like to mark a point on the wall and stab it, but that doesn't sound like an option.  The next best way is the tennis ball method - poke a hole in a tennis ball and put a rope out one end.  Toss the ball over something it can safely hang over (a basketball goal or maybe a hook in the ceiling) and tie the other end of the rope to a point in the middle of the rope, creating an adjustable loop.  You could also just fix the length and make a more permanent hanging tennis ball.