The double haul is not as hard as you think. Go to Youtube.com and loop-up Orvis double haul.

I rarely use it. Usually only useful in long casting situations. It's tough making the transition from single to double haul. I use it so rarely that I have difficult calling it up when I need it. Double hauling is also very useful in heavy wind when a heavy rod and even heavier line is necessary. And lots of muscle. I can't remember when I last used double haul with a light outfit though. I'm 6'1" and I can usually get what I need from single haul.

I learned it with a rod in my hand casting on an indoor tennis court but it seems most of the pros today want to teach it going through the motions without a rod in the hand first. Also, make sure your line is good and clean (always a problem on tennis courts because there's so much fuzz from the balls laying around). You'll want to be using a shooting taper line (not double taper and definitely not single taper). Keep at it, you'll get it eventually.