The easiest way I've found is simply getting a small drop of water exactly on the ding - and let it sink in and dry. I take a small piece of paper towel and twist it into a point, barely dip the end in water, and squeeze a drop out of the twisted end's point.
After it soaks in and engorges the wood, feel the spot - is it still indented? If so, repeat as often as necessary. When the area feels flush, used some extremely fine grit sand paper or steel wool in the general area, being careful not to take much wood off so to avoid creating a flat spot/area. A good ding may take 3 to 4 applications like this.
There are other processes too, like steaming, but I never needed to go to that length. This is so easy IMO.