I am not by any means an expert, but I have found that on long flats where I need to go as straight and fast as possible, I actually weight my BACK foot. Or maybe I am just weighting evenly, and to me it feels like I'm too far back since I'm used to being further forward when riding more dynamically.
Basically though I take a lot of weight off my front foot, and crouch down to limit wind resistance, and rest my elbows (and therefore my entire upper body) on my back knee. This helps keep my hips/chest/shoulders parallel to the board (which is much different than how I twist them when carving), and I can also get very slightly on either edge and go ALMOST straight. In my mind it is preferable to be going ALMOST straight (but on an edge)... true you have to cover slightly more distance, but you decrease the friction between the board and the snow by more than enough to make up for it. I find it easy in the position I described to rock from rail to rail every so gently and give the illusion that I am going straight, but keep greater control and speed. Plus, I hate being 100% flat because I am always paranoid the wrong rail will catch first if the board starts drifting.