[Courtesy of Mark Drela drela "at" orville.mit.edu, August 2000]
For the HLG newbie, a good way to improve sanding accuracy is to use a LONG sanding block. The 22" Great Planes bar is perfect. Alex's has them I think. A 1x2x24 pine bar is also good, of course. A long bar allows better control of fore-aft "rocking" which leads to rounding of surfaces which are intended flat.
For instance, a good way to get the blank flat is to hold the bar far away from the balsa, and wrap your thumb and pinky just under the bar so they touch the table and hold it off the surface at the height of the balsa. It's easy to see if the long block is parallel to the surface. A shallow light shining under the bar might help. The three middle fingers apply pressure to the bar. The leverage is lousy, but if you have to press hard then your sandpaper is too fine. I start with a razor plane or just 60 grit, and quickly switch to 80 for most of the shaping. I switch to 150 grit with about 5-10 mils to go. Then 220 and 400 for the last few swipes, and also for the sanding sealer. Final polish of the last sealer coat is with DuPont polishing compound on a cotton rag.
Back to sanding... One other technique is to sand two wings at once side by side, with the bar straddling them. You can team up with someone or cut your one wing blank in half. By repeatedly swapping the wings as you get close to the right thickness, they will end up flat and rectangular in X-section. Tape a strip to the table against which the tips are butted when you swap the wings. This makes the spanwise alignment fast and repeatable. Sanding two whole wings together like this can be faster than sanding one, since you have to stop and measure much less often.
If you cut the wing blank in half to taper it, I suggest gluing it back together again for sanding the airfoil. The butt joint should be perfect if the saw cut was clean! It's easier to hang onto when it's in one piece.