Rotate seating positions of family members so that all cushions are equally used. It's not uncommon for individuals to have a favorite spot when lounging on the couch, but this is a major contributor to sunken-in couch cushions. By sitting in different positions and on different parts of the couch, you'll eliminate excessive wear on individual cushions.
Flip removable couch cushions once a month to distribute wear evenly over both sides of the cushions. In addition to keeping the fabric looking nice, it will keep the cushions from sinking in. It is important to rotate the back cushions as well, as these can also become sunken in.
Place throw pillows on the back of the couch to protect the cushions there from sinking in (particularly if these cushions are not removable). Sitting against a few comfortable throw pillows will help to better distribute weight across the couch.