The old thin-bodied stout is definitely one of the more frustrating flaws for this particular style. From what I have observed, this flaw is often associated with stouts that are brewed in an attempt to mimic the famed dry stouts of Ireland like Guinness, Murphy’s and Beamish. These draught stouts all are dispensed using nitrogen and much of the body is directly related to the method of dispense. Take away the nitrogen dispense methods and the result is a thinner-bodied dark ale with the alcohol content of a light lager.
So, body-building tip #1 is to use mixed gas dispense methods when brewing dry, Irish-style stouts. Bottle conditioning this style or serving it with normal draft equipment is not going to result in the type of stout served at your favorite Irish pub.
The other frequent cause of thin-bodied stout, especially when styles other than dry, Irish-styled stout are considered, comes down to malt selection. It seems that many stout recipes contain pale malt, roasted grains (roasted malt, roasted barley, or a combination of the two totaling about 10% of the total), and perhaps a few other malts thrown in for good measure. Full-bodied beer, in general, begins with a grist bill composed of a variety of grains that have complexity of flavor.
Body-building tip #2 is to beef up your grist bill. If you are using very pale malt, or pale malt extract, as your base ingredient consider adding high-kilned malts like Munich, Vienna, amber and/or brown as a substitute for the pale malt. I also like adding crystal malts to my stouts to add some mid-palate fullness and a bit of sweetness in the finish. The type of crystal can be changed to bring different flavors into the fold based on your personal preferences. I like using darker crystals when I want the raisin and dark toffee notes associated with these sorts of malts. Lighter crystal malts bring a less obvious flavor to the mix. Another classic grain to add to stout is flaked barley or oats at a rate of 10% of the total grist. These grains are both rich in beta-glucans that add palate fullness. Oats are also known to add a silky, sometimes described as oily, mouthfeel to beer. And both grains slow down wort collection because they increase wort viscosity.
Sometimes beers lack body because the wort strength is simply too low to leave enough extract behind following fermentation to give body. Body-building tip #3 is to increase residual extract by adding more malt to increase the wort original gravity and/or by using higher mashing temperatures to reduce wort fermentability. You can enhance the perception of the increased residual extract by reducing your carbonation levels and serving your beer at temperatures that are warmer than the typical refrigerator. Hopefully there is a cure for your stouts in this answer.