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: This is based on my observation on StackOverflow. There is, however, one important difference between StackOverflow and AskUbuntu: SO has so much traffic that questions only stay on the front page for about 10 minutes after the last answer.