I use 3M's 100 or 150 grit. Remember though that sandpaper can't fix all. Many drywall flaws are there because someone skipped a coat of mud. If, after sanding, you can feel a dip with your hand, that needs to be filled with drywall compound.

The sanding should make a surface dull...Too fine a sandpaper will "load up " quickly, maybe overusing results in a shiny surface ?? The Paintguy is correct,
Mud, and this is easy but laborious, so poor contractors stop short of a good job.

Thanks guys! I went to the store to buy the sandpaper, prepared with my numbers, but they said fine, med or coarse! lol just my luck. I got the med sanding block. :-)