Always back sandpaper with a flat sanding block when hand sanding flat surfaces such as tabletops to avoid grooving out the softer, spring growth, areas of the wood. (It’s rarely necessary to use a sanding block when sanding between coats of finish because you’re using very fine grit sandpaper and not sanding through the finish to the wood.)
You can use a commercial sanding block such as the Zip Sander, TheFinishingStore.Com, or you can make your own sanding block, from a piece of wood with 1/8-inch gasket cork glued to the bottom.