i've found that the best way for me to go upwind in light conditions is to have the kite quite high (above 45 degrees) and don't go any faster than you have to keep a plain going. I've found that the faster i go, the more the apparent wind comes around to the front of you in effect sending you further downwind. i don't lean back to much, so i can keep the board as flat as possible, and use the fins to stear me upwind.
Hope this has helped