Before Sharpening, Flatten the Back of Chisels and Plane Blades
It's impossible to get a sharp edge on a chisel or plane if the back of the blade isn't flat. If you're using glass and sandpaper, press the back side of the blade against the sandpaper and move the blade back and forth, being careful to hold it perfectly flat. Sand the back of each blade until it's uniformly shiny along the cutting edge. Do this with each grit as you move through the progression from coarse to fine. Use the same technique with natural or diamond sharpening stones.