Alright, I feel irresponsible for failing to post this earlier, but today is officially the unofficial "That's what she said!" Day. This day is unfortunately more than half-way done, but I would say most of the good comments on which to use the line will happen later in the day.
In the TV show "The Office" Steve Carell gives some stunningly bad examples of how to use this line, so go out and share it with the world.
An example might be... (Sitting in Artificial Intelligence learning about Alpha-Beta Pruning)
Student: "It seems like it should be really hard, but it isn't!"
Me: (Grin) "....That's what she said..."
Because it is so late, you're going to really have to get on it if you want to make any impact