I'm fortunate to live around some rolling hills...for trainig, I simply make a point to get out of the saddle for the uphills if they are going to take about 30 seconds or less....then recover on the down hills...I think it has really helped my out of saddle endurance...

Or if you want to go crazy and injure yourself, I saw an interview with Alberto contadore where he said he would pedal out of the saddle for 20 mins!