Sometimes adding a little extra fat helps, too, depending on the dish. Start with the sugar or honey, like AJ suggested.

A tiny pinch of sugar. Stir it well, then let it sit for a few minutes. Then taste it again. You can also use a few drops of honey. But be quite judicious, lest you over-do it. For a salad dressing that's too acidic, I find that a bit of water does the trick nicely. ;o)