There was an interesting thread on a perfect marathon stroke with a double bladed wing paddle in the Texas Canoe and Kayak Racing forum.
Tommy Yonley wrote a few very good tips. They are quite general, not specific to wing paddles.
The most important thing about your stroke is to pull hard (of course, in such a way that propels the boat forward). A typical description of "perfect technique" sometimes omits this critical bit of information. The whole point of everything about technique is so that you can pull harder for a longer period of time.
You want to paddle in such a way that your elbows and wrists stay at basically the same angle (a slight bend most of the time, of course, a more advanced paddler will straighten his bottom arm to reach as far forward as possible, but for just getting the general idea, try to keep your elbows basically at the same angle). The reason for this is that if your elbows are bending a lot during your stroke, it means that you are pulling with your biceps--which are not a particularly big muscle (compared to your lats) and thus will tire out quickly. You want to use the biggest muscles to do the most pulling so that you can pull harder for longer.
Now sit with your elbows mostly straight and your arms extended out in front of you, as if you are about to paddle--but have both wrists at roughly the same level. Try to make the motion of taking a stroke keeping the "top" wrist only a few inches higher than the bottom wrist. Either your paddle will make a rediculous sweeping motion, or you will bend your bottom arm--and either way, you can't pull very hard for long. The point here is that you can't really do a decent stroke with the paddle very horizontal. I don't think it is important to be perfectly verticle, but the goal should be to get as vertical as your body reasonably allows.
You want to paddle with the blade perpendicular to the water such that the water resists the motion of the blade as much as possible--and propells your boat forward. Many people describe this as the paddle being "locked" in a certain spot in the water. It is surprising that when people are learning how to paddle, they don't always focus on this (especially when they are distracted by trying to do all kinds of rotations, and letting the wing "wing out" from the boat). This is really important. The more resistance you feel when you pull, the better you have judged the correct angle for your blade to be in the water.
At the beginning of your stroke, while your blade is entering the water, you are at a very important time. If you begin pulling on the paddle while only the tip of the blade is in the water, you will cause cavitation that will continue as your blade goes deeper and deeper into the water and causes your blade to more easily slip through the water--a very bad thing.
On the other hand, if you simply relax, and let the paddle slowly sink into the water before you start pulling, you may find that your paddle has already reached your waist before you actually start pulling--once again, your stroke has been wasted.
The best thing to do is to make an effort to quickly burry the entire blade well in front of you, and then pull--but not in a way that seems "jerky". Getting this done, where you neither cause cavitation nor miss a large portion of the "power phase" of your stroke, requires deliberate action and some skill on your part. I think most paddlers refer to this as the "catch".
If your stroke makes any gurgling noises whenever you pull hard, you are not burrying your blade deep enough before applying power or otherwise screwing up your stroke.
Pull hard and twist during your stroke. You will notice that at the end of your stroke, your twist has nicely set you up for the start of the next stroke on the other side (do not begin twisting in the opposite direction until you have gotten your blade into the water).