I haven't really sailed wave boards or anything like that, but sailing upwind is one of my stronger angles given the amount of RSX racing and training that I do. I've got a picture here of my upwind stance, and I'll try to explain it.
One thing to remember is to keep your body comfortable and healthy, so keeping your back straight is important. Next, I like to keep my front leg straight, and my back one bent. Locking the front leg helps to keep the windward rail and the nose of the board under control, stopping the board from 'flying'. The back leg then controls the power to the fin - more pressure and more height, less pressure and relax the load on the fin. It also helps to recover quickly from a spin out, as you can then pull it up under your hips effectively. Working up the body, my hips are rolled forward. This means that my hook is pulling down more on my harness lines and helps me to take the weight out of my arms (well balanced harness lines will help lots). My head is looking upwind, keeping an eye out for gusts and waves that could upset the balance I have created, and gives me time to prepare for them and allows me to adjust quickly. But the most important thing by far (in my opinion) is having the clue of the sail cranked in tight, and the foot pulled back as close to the board as possible. Once it is in and back (like in the picture) I like to imagine it being glued there. This creates a great flow over the sail and generates lots of speed, which means that you can push hard on the fin and it will have more 'traction'.
The RSX is 220L board with a 66cm fin and a 9.5m sail, and in the picture im sailing in 15-20 knots, but I use the same technique on my 90L Isonic with the 5.6 in 35 knots.