I flatten them with a belt sander, assure a great fit, then run a toothing plane blade, just the blade, held like a card scraper, down both surfaces lengthwise. The grooves ensure the joint wont be starved,(I use Smooth On epoxy) and since the toothing plane blade edge is shaped/sharpened very slightly convex, the glue joints along the edges are perrrrrrrfect... every time.