The longer the lens, the more shallow the DOF at any given f-stop. Opening up the lens (smaller f-stop number, larger opening) and moving closer to the subject also also gives more shallow DOF. Using a camera with a larger film or sensor area, also gives more shallow DOF. That is why 4x5 view camera guys are always trying to get way into the f16 and higher range, while those using 35mm can get away with f2.8 on a 300mm, which is very popular with fashion shooters.
More than simply thinking of DOF when thinking about a longer lens, is the perspective of that lens. Sure, you could get very little DOF with a 50mm (in 35mm format), but you'd have to move in so close to the subject (for a headshot) that you would get a larger nose and features. For this reason, most 35mm shooters use at least a 105mm for headshots (sometimes even longer). In 1.5x digital terms, that would be about a 35mm focal length. If you are using that kind of digital, your 50mm is not great (its only the equivelent of a 75mm). An 85mm would give you approx 125mm effect, which would be perfect. When people talk about compression, they're talking about the flattering effect of the "compressed" image on a face especially. It makes the features smaller and just looks nice. Here's one with a 300mm, probably at f4.
Andy Pearlman Studio