Technically, your DOF is probably not going to change with longer foccal lengths. That's because you need to increase your distance to subject for the same framing when you go longer (as Ira and I have both pointed out).
For example, if you go from 50mm to 100mm, you'd need to shoot from twice as far away for the same framing (percentage of the frame your subject occupies). So, DOF is going to be the same.
But, the appearance will change (you'll have a more compressed background because of your shooting distance). You'll see this referred to as perspective.
That can make the out of focus areas more obvious, and give you the illusion of a shallower depth of field (as compared to using shorter focal lengths and shooting from closer distances).
So, with some models (especially thouse with wider apertures available on the long end), it's best to shoot zoomed in all the way to maximize the impact of a shallow DOF).
Again, frame tighter. Go for the tight head and shoulders versus the full length portrait and you may be able to get what you're looking for in some conditions with a non-DSLR model.