Unfortunately the most efficient way to farm XP is to rephrase existing simple answers on easy questions while the question is still hot. Long detailed answers on difficult questions are not worth much because too little people are interested in them.
Edit: The above commentThis is based on my experienceobservation on StackoverflowStackOverflow. There is, however, one important difference between StackoverflowStackOverflow and AskUbuntu: SO has so much traffic that questions only stay on the front page for about 10 minutes after the last answer.