The most reason people use a sander is that they want a smooth surface. But before you can sand surface smooth, you should level it and make sure it is free from defects. If you have a belt sander or even better a drum sander, it will be much easy to attain a flat defect free plane.
Leveling means you sand below all defects, and end up with a flat and consistent surface that’s ready for further smoothing. To maintain flatness, you have to sand consistently and evenly across the whole of a surface down to just below the depth of the defects. Quick stock removal is best accomplished by leveling in multiple passes.
Most of us have always been taught to sand wood in the direction of the grain, however this isn’t the best technique in the case of stock removal. In that case, such as abrasive shaping or leveling, you will remove stock more rapidly when you sand at a 45 to 60 degree angle across the wood’s grain. At this orientation, the sand grains aggressively remove large bundles of wood fibers, rather than just slicing between the bundles, as they would when sanding with the grain. An angles sanding also removes stock more quickly than perpendicular sanding across the grain, which slices fibers but doesn’t remove them as well.
You will work more efficiently when you change sanding direction after each pass because that way you will be able to see all the scratches from previous pass. Take the first pass sanding in one direction at an angle across the grain, then change grits and take the second pass sanding at an angle across the grain in the other direction. If you need to use more grits, continue to alternate directions on subsequent passes. On your final smoothing pass, sand in the direction of the grain to remove the cross grain scratches left by the previous pass.
This technique works great with belt sanders or when you perform any kind of straight sanding action like hand sanding with a sanding plane. It even works when running stock through a drum sander. Instead of feeding the wood piece in the direction of the grain, you feed it at an angled direction.