to add an acid! most soups do not contain any acidic tasting ingredients. adding something like apple cider vinegar, rice vinegar, red wine vinegar, or a red or white wine itself, will usually do wonders for the flavor of your soup.
how much to add? usually more than your think. of course do not go crazy, simply add a little vinegar or other acid a little at a time, tasting along the way until it you think it tastes good.
of course there are some soups, like a tomato soup which can be pretty acidic already. in this case, do not add more acid! rather, focus on step 3 below.
step 3: do 