Having a more substantial mouthfeel is generally associated with a high FG. The more sugar remaining after fermentation, generally the thicker the beer. Similarly, a thin beer will have fermented nearly all of its sugars.
To attain a higher FG, you need more unferementable sugars, which are exactly what they sound like! When you mash at higher temperatures (156F+), less fermentable sugars are created. When mashing at lower temps (148F) you create almost only fermentable sugars.
mashing at 148=thin beer
mashing at 156=thicker beer
flaked barley, and carapils are known to produce better mouthfeel and increase head retention.
Also, using yeast with lower attenuation will give a thicker beer. low attenuation=ferments less of the sugars available. American yeasts generally have higher attenuation, which results in a thinner beer when compares to its UK counterparts